How can I tilt up a stem of my bike's handlebar which is held by two bolts?
Adjustable stems follow a couple different design schemes, all of which involve some interlocking toothy bits that hold the angle in place. If the angle isn't changing, the toothy bits are still locked together.

There's probably a bolt on the underside of the stem extension. If so, that's what locks the angle adjustment in place. So you loosen the side bolts, loosen and adjust the underside one, re-tighten underside, re-tighten sides, in that order.

There's another type of adjustable stem without the underside bolt, where the side bolts are tightening down a pair of toothed plates that hold it all together. That doesn't appear to be what you have, but in the case of a recalcitrant one of those, you would just need to gently pry the plates free.