S6 Software update failure

[AC Question]

Bought off eBay. It was locked to O2, I got it unlocked. I switched on the phone and it said the software needed updating . . . it failed to update because it said that the phone had been modified and that I should download Samsung Smart Switch, I downloaded that and went through the process which seemed to be happening but when it finished it still showed the message ’software update available' 'current version PDA:QFM latest version PDA:QG4' I again clicked the update button etc., and it just goes round and back to ’software update available' 'current version PDA:QFM latest version PDA:QG4.' I called the Samsung helpline and they said that the software had been 'rooted' and that to solve that issue I need to take it to an official Samsung centre for 'repair.'