I just purchased a used iPad and the old account is still linked to it. How do I get rid of that?
Plug it into your computer, open iTunes, hit the RESTORE button. When given the option, select "set up as a new device". this will make sure it's linked to your pc rather the previous owners.
Best practice when buying a used iOS device is to completely reset it either via the restore option within iTunes or on the device itself via "Settings > General > Reset > Erase all Content and Settings".
If you have already personalized the device and only want to change the Apple ID linked to it go into "Settings > Store", click on the Apple ID at the bottom and click on "Sign Out"