Assuming you are a Windows user and are not hugely technically competent, download and install
Everything
Everything is a simple search tool that uses your hard disks own indexing to instantly search, well, everything.
You can use it to find, with no knowledge of where twitch / classic / FTB / MultiMC etc. typically store things, where your various Minecraft profile folders are. You could use your regular hard drive search, but its slow, oh so slow.
All Minecraft launchers store their instances as a very similar Minecraft profile folder, typically the instance folder contains a sub folder called ".minecraft" or just "minecraft", and that folder contains the various Minecraft folders that make up a typical profile: "mods", "config" etc. and most crucially "saves". Use any of these folder names as key phrases in Everything and you will quickly find where Twitch or FTB or whatever has stored the saves for your various instances and then be able to copy the save you want.