please pay attention that screws are native to metal pieces otherwise you are going to be a stupid like me trying to unscrew unsuccessfully until I reread the guide
for me this was the hardest part especially the two 14.5mm that dint want to come out. keep turning the screw and slightly pressing the case down, once the screw is a little use try using finger, screw and a non metallic point to try to push them out. they arent tight but they are very hard to unscrew i almost gave up, but believe me they come out with patience
please note the position of the screws because when you are reinstalling them you might get confuse. this ones go below or under (sorry English inst my first language). There are two screw holes this steps are for the ones below.
I had no problem at all in this step, the left side was already unlocked when I open the MacBook, the right side was a little tricky but i was able to pull it out in less than a minute
it was really easy, i had no experience whatsoever installing or unistalling mac parts (or pc either). it just took me less than 25 minutes, and i just finish and Im typing from my new upper case keyboard.