how can I edit a document that is saved on my USB?

Connect this USB drive to your iMac. It should mount on your Mac's Desktop. Once mounted, use the Finder app to locate the file you want to edit. You should now be able to use your preferred editor app to edit it.

You can open the document using the app that created it and make your changes (you can move the app back to your computer to do so). If you saved it as a .pdf though, you won't be able to do that, so you would need to open the original version.

Without knowing the document type and what you mean by "my USB" we cannot answer the questions.

If you mean on a USB flash drive, I wouldn't. I'd move it to a real HDD or SSD for greater security and reliability.

Assuming you're backing up of course. If you are, most likely the files on your USB drive are not being backed up.

If you mean an external USB HDD or SSD, that's better for security but it's still unlikely the files are being backed up.

How is your USB formatted.