Backup tablet before repair
In terms of non app data, just plug it in to a computer, and move it over. Just go to your device in an file manager/explorer, and copy the pictures, DCIM, Download and Music folders to your pc. Then, if needs be, you can copy them back.

App data will be different. Assuming you have a Google account, just sign in to it again when you get it back(if it has been reset), and it will re-download the apps automatically. If they have cloud save (most games do) it will re-download his progress.