Talk to your new class trainer in which ever inner city zone of Qeynos they are.
If you are a neutral class (dirge/troubadour, berserker/guardian, fury/warden, warlock/wizard), then you can choose whether to stay your current class or switch to the other associated class.
If you are an alignment restricted class (e.g. paladin/shadowknight, conjurer/necromancer, etc.), then you must talk to the trainer for opposite alignment to your class.
All Fighter trainers are found on the steps to Qeynos Castle in North Qeynos except for Brawlers (Bruiser becoming Monk), who report to Khai Lightfall ( 770, -20, -375 ) Copy/waypoint 770, -20, -375 in the Dojo in The Elddar Grove.
You will be asked several times if you are sure you want to be your class to confirm your choice.