If the threads are corroded or siezed, apply some penetrating agent such as light oil/solvent (WD40).
Leave the pedals for a day or 2 then remove using a long handled wrench.
The non drive side has a reverse thread so rotate clockwise to remove.
The best technique for pedal removal is to stand the bike upright and apply the brakes.
Turn the crank arm to face forward and up.
Apply the wrench to face rearwards and up.
Press down on the wrench with your foot and apply weight.