Setup as new iPad?
1. Turn on iPad
2. Set Language; tap arrow on top right
3. Set Country; tap Next
4. Enable Location Service; tap Next
5. Setup WiFi (connect to internet)
6. Setup as new; tap Next
7. Sign-in with Apple ID
8. Agree with Apple Terms and Conditions
9. Enable iCloud
10. Enable Find My iPad
11. Register iPad
12. Start using iPad
You can't go back without erasing the iPad and starting all over again. What customization are you referring to that you can't do now that the device is setup?
In any event, if you feel the need to start all over again go to Settings>General>Reset>Erase all content and settings and that will wipe the iPad completely and you will have to start all over again.
No, I have an existing iPad and I was just installing an update. Resetting or starting from scratch is definitely overkill. After I installed the update, it went into a configuration "mode" and asked me several questions. In that, it asks me my Apple ID. Since I was,doing this for a friend, I didn't immediately know the Apple ID, so I came back to it the next day. However, there was already some usage in the meantime and I don't know how to go back to where I was.