The belt is non adjustable. The belt is routed around a spring tensioner idler pulley which keeps a constant and proper tension on the belt. If the belt is squealing at start up then it could have a worn belt, belt debris in the grooves of the motor pulley, excessive load in the drum or possible a friction problem with the drum rotating.
Without knowing the model number of your dryer I will not be able to provide any model specific information.
Resubmit your question with the model number and then I can provide the model specific information how to access the belt, motor and drum rollers.