Use a Philips Head #000 (PH000) screw driver to remove the screws. I made the mistake of using #0 and #00, using a common precision screw driver set from Walmart, which stripped it for the tougher front screws. HP uses thread locking glue so it is hit or miss for successful extraction without the proper screwdriver head. If the screw does not come out easily, stop! If this message is too late as you have already stripped it, screwdriver just spins freely and you can see silver, use a Microbit Screw Extractor (the smallest one in the set) with a power drill. Install the bit, set the drill on reverse, set the lowest speed or infinite with a slow press. Pressing down firmly on the screw head start with the side having the big teeth. If it bites the screw will extract. In my case I did not even have to use the other side with more threads. As for replacement screws, there are laptop screw sets you can buy from Amazon and such. For the 2018-2020 Omen 15 the outer case screws are M2x6.0.
(PAID link) Microbit Screw Extractor (Amazon)
(PAID link) Assorted Laptop Screws (Amazon)
(PAID link) Screwdriver set containing PH000 (Amazon)