Bend the piece that goes behind your ear at around 1/3rd or 1/4th of the length (right where it bends down to hook your auricle). If glasses are made of plastic or similar material you might want to heat them slightly with a hair dryer to make them a bit more flexi.
I bought a few pairs of glasses during lockdown, obviously bought online without trying.
I find they slip down my nose compared to other glasses, any suggestions on which bit I should be bending to stop this?