angle gauge

if i am doing an angle of 90 tightening a stud

and i go over the angle is it okay and safe to loosen it back off to the 90 or will i have to replace the stud??

If it's the last stage of torque and you exceed the recommendation then this will have permanently deformed (stretched) the fastener and will compromise clamp. Of course it also depends on how far you've gone. 5-10 degrees is unlikely to be a deal breaker 45-90 degrees too far and the fastener is probably toast. If this was just the first stage and you have say another 90 deg to go then it's not an issue. Back it off and continue.