What a great question!
The bad news is that for the technician to do their job, they normally need your PIN. Once they have your PIN, you have completely lost the security battle.
- Backup your device (to the cloud or computer).
- Reset/erase your device.
- Hand it over to the tech for service.
Note: assuming you don’t have a hardware problem, steps 1 & 2 get you halfway to fixing most problems yourself. To go the rest of the way:
3. Reinstall current OS.
4. Install your apps from fresh downloads.
5. Migrate your data to your device.
I’m most cases, you are now fixed.