Suppose my mobile phone has gone bad. So I gave it to a mechanic. So what will the mechanic do with the phone to "make it return to its original state"? What is this process called?
While it's possible, but highly improbable, that a native Anglophone will call someone who repairs cellphones a cellphone mechanic, in American English, we would normally call such a person a cellphone technician. The word mechanic is usually used to refer to someone who repairs large machines rather than small pieces of electronic equipment.
If you give your phone to a cellphone technician, the person will fix or repair the phone. Those are two names for "the repair process". Sometimes that involves replacing a module or maybe even soldering a connection (rare, because electronic devices are not made to be repaired these days; they're made to have little detachable modules replaced or to have the motherboards replaced because doing anything else takes too much time & costs too much money).