How to fix a racked/sagging exterior wood entry door?
The quick fix is to install a diagonal cable/turnbuckle assembly from the swinging lower corner (where the vertical lock stile meets the lower cross-rail) up to the opposite corner (where the hinge stile meets the top cross-rail) and tighten the turnbuckle until the door is more square.

The other fix it to rebuild the door and tighten, glue up and clamp all of the stile to rail joints.