How do I adjust the tension on a peugeot expert alternator?

Raise the vehicle and secure it on axle stands or similar. Remove the front right wheel. Remove plastic inner arch covers. You should see the tensioner mechanism in front of you. There is an Allen headed socket bolt through the slot in the mech - loosen this slightly. Underneath the vehicle is another that will move the tensioner in the slot to tighten or loosen the belt. Adjust this to correct tension then retighten the bolt through the slot and refit everything.

HTH

Roy.