I want to edit a piece of clothing's type. It is a piece that is in swimwear, but I want it available in sleepwear.

I was recommended another program before on Loverslab, but that program I have no idea why it doesn't work and the person who recommended it didn't know either because he recommended it online; that said, I'm quite a boomer when it comes to technology, so if you guys have any program recommendations, the more user friendly it is, the better.

S3PE is just fine, but I don't know about a dodgy reupload of it when the original hosting is perfectly available?

But S3OC is a little more user friendly for simple tweaks like this. If it's a piece of EA clothing, choose CAS Part from the Cloning menu and find it in the list. If it's a piece of CC, you can open it with the program and select the CASP.
Then hit "Clone or Fix", untick 'Renumber/rename interally' unless you want to make it standalone, and make any changes you want to the category flags that will show on the right-hand side. Press Start and save the new package, and you're all done

So new development. The S3PE version you linked me worked, but the S3OC when I run the program. The program doesn't pop up.

Anyways, since the S3PE on my computer now works, how do I access EA stuff with the S3PE?

EDIT: Also, I realize S3PE only work on Package files and doesn't work on The Sims 3 Custom Content files. Is there a way to edit The Sims 3 Custom Content files?

Both programs have the same requirements so it's a little strange for one to work and the other not to, but you could try the version that the auto-link on your comment goes to instead. S3PE is usually used in tandem with something that can get the packages for you, so it's a little trickier by itself.
There is also EasyCasp, which is a little more limited, but I think it would be just fine for what you're looking for. That one can edit things you installed through the launcher as well. Modifying a .sims3pack with the other tools would require converting it to .package format.

S3OC downloads, but when I run it, the opening icon pops up, but then nothing. Also, is there a way to also edit "The Sims 3 Custom Content" items? That way i can also edit content from places like TheSimsResource?

.package and .sims3pack are the formats that all custom content comes in. If you don't have file name extensions enabled on your computer, you can identify them by their icon. With S3PE installed, the former will have an icon of a green plumbob over a cog. The latter has the same icon as the game launcher.

Once a sims3pack has been installed through your launcher, EasyCASP can see it under 'installed content'.
Packages are the ones placed manually in the mods folder and are typically edited with S3PE instead.
There is some more information about the differences here, if needed.

Instead of messing with sims3packs, it may be easier to uninstall it via launcher, convert to package using Delphy's tool and then change categories with s3pe. In s3pe, it is just a matter of select the CASP, and click GRID at the bottom. In the new window, click the arrow next to Clothing Category. Set what you want to True, and what you do not want to False. Save and done.

This is the way I do it as well, but I thought I'd start with whatever OP is most familiar with and go from there. Especially with the request for user-friendliness, as S3PE is a less straightforward interface to use even if it's the more powerful tool. And if they need to change EA content too, EasyCASP is... well... easy
But yes, this is definitely the tidier way to install and modify CC, if interested in doing so.

I think I had EasyCASP when I first started playing, but many said that converting to package and merging was better for game performance, so that is what I then began doing. I now only install worlds as sims3pack, after a couple I tried to convert did not work.

You sir are a miracle worker, thank you soo much

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I totally agree, which is why people come to MTS, one of the oldest Sims forums, and ask those of us who mod, or create CC, use the various tools, and can give multiple perspectives on ways to reach the end goal of the questioner. There is often more than one right way to do things, and people can try each, and find what works best for them.

I suspect that comment may be a bot- I usually err on the side of caution with these FirstnameLastname usernames that pop up here and say something incredibly generic and slightly off within a day or two of signing up. I've seen a couple of different ones use the same reply, as well as one that edited one of the generic comments to later contain a scam link.

No offense if JennaBusse happens to be a real person who was just busting to share a blanket statement in a thread that had been resolved, but it's very suspicious

Perhaps a bot, but I thought my reply was honest and accurate enough. Besides, some bots are really nice!