Hi i want to change my "Bright Box2" to faster WIFI i.e. AC but i have been told that i can't use ADSL2 as i have fibre is this correct if any one can tell me
Ignoring the confusion between ADSL (up to 17Mbps) and VDSL "fibre" (up to 38 or 76Mbps) in your post, so long as you can find the broadband login and password on your router (NOTE: This is NOT the same L/P as you use to access the router's admin interface or your WiFi) then you should be able to just buy a third-party router and get it working.
The relevant details are explained here and you should have the password from when you first signed-up.
Now for the avoidance of doubt the simplest solution would be to buy a router that has both ADSL and VDSL integrated. Personally I rather like the TP Link VR200 AC750, although there are plenty of faster models.