Remove the head from the lacrosse stick. There are three ways your head may be attached to your stick. It may be attached with a single screw through the base of the head into the stick. It may be a screw-on type. Or, it may be held on by pressure and you will need to get someone to help you pull it off (one holding the stick and one pulling the head).
Un-string the net from the head and place it to one side.
Place the head in a microwave and microwave it on high for 30 seconds only. Check the head and see if it has become pliable. Repeat this until the head is softened.
Remove the head from the microwave and press a hard ball into it. The ball should be the size that you need to stretch the head to be. Let the head cool and harden, pop the ball out and restring the net. Re-attach it to your stick and you are ready to go.