how do i re root my samsung s3 to factory settings?

hi there i was having trouble with my galaxy s3 its just over a year old now and started to not charge properly and all of a sudden my phone completely died so went to get it fixed. when it came back i now cant use my internet on my phone only wifi and i cant update my phone. its also showing vodaphone apps when im on t-mobile. it seems that they have rooted my phone to get it working again. is there any way to restore my phone to original factory settings? and getting working properly again? im new to these websites and dont know much about phones so any help will be much appretiated