Replace the gas struts. They are available from Volvo or aftermarket. A common brand is Stabilus. Instructions are to be found on volvospeed/Bay13.

It is not hard but you will need 2 people - your partner will hold up and overextend the door so there is really zero tension on each strut while you remove it. The door is very heavy and will exert tremendous force at the hinge point. The struts will not come free unless someone actively overextends the door, so a prop, etc., will not work.