Open the Pella casement window.
Locate the window sash keeper. The sash keeper is the locking mechanism that the window lock hardware fits into. If the window opens from the left-hand side, look for the sash keeper in the bottom right corner; if the window opens from the right, locate the sash keeper in the bottom left corner.
Remove the two screws from the sash keeper, using a flat-head screwdriver. Reinstall the screws into two different holes on the sash keeper. Shut and lock the door, checking to make sure it's tight.