There are similar questions here, but this one is about a phone that doesn't turn on. I can't reset to factory.
What should I do? Change my Google password? What information will the technician have access to?
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You might want remove the external sd card before giving the handset.
It's safe to change your google account password associated with that phone.
Basically what they try to do IS a factory reset, in which case, you shouldn't be worried about the stuff on your phone.