how to remove a stripped screw screwed to the motherboard?

Removing stripped screws usually involves pulling up on the screw while unscrewing. Not easy in some situations. Try sliding a small flat blade screw driver or thin piece of metal under the screw head and forcing it up while you unscrew. Some small screwdrivers have a keeper on the blade used to hold small screws as they are being put in, bit also work removing stripped screws. Never saw one for Phillip's screws

If the screw head can be lifted enough, try to grip the head with a good pair of pliers. Then pull up while you unscrew it. Sometimes a lot of tension is needed to get a thread to grab. Also try to pull the head sideways while pulling to give it a good chance to bite. If this doesn't work, you will need to drill the screw out or grinde the head down using a Dremel type tool.