i have a special lever tool that fits into the square slot on the tensioner to be able to pivot it, but you can do it from underneath with a prybar its just fiddly, to remove the tensioner completely you have to remove the lower gearbox mount and the engine mount and jack the engine up as far as you can to get to the allen key holding it, there is literally just enough room, if you are just changing belt iirc by just loosing the tensioner slightly you can tease the belt around it and the other pulley near it instead of removing the whole tensioner