I've always believed the past tense of sync ("I sync my phone with my computer") to be synced ("I synced my phone with my computer yesterday"). This question would seem to suggest either synced or synched. I have a friend who insists the past tense is sunk. I'd appreciate if you could make this clear for me.
The past tense is "synced". "Sunk" is the past tense of "sink" which sounds the same but is a completely different word. "Synced" appears to have made its way into dictionaries:
Edit: As others have pointed out, "synch" and "synched" are acceptable variants.