There's a file called Users which has my old user and all the files on it, and I was wondering how to make it my default user again.
You can't. Even though that was you on that PC, this PC can tell it wasn't created by the same user account.
You can only have one Users folder and you already have one on your C drive. Its a chicken & egg thing, your user cannot exist without a User folder for it to be in.
You can't use that folder now. You can maybe copy some of its contents but you are better off not trying to use anything in it.
to avoid permissions problems, try this
go to settings/update & security/recovery
under advanced startup, click restart now button
this restarts PC in a blue menu
choose troubleshoot
choose advanced
choose command prompt
type
notepad and press enter
in notepad, select file>open
Use file explorer to copy any files you need to move to USB or hdd