Snap the blinds out of their brackets to take them out of the window.
Remove the end caps from the head rail.
Note the condition of the friction clips inside the head rail. These clips sometimes break and must be replaced. Replacement friction clips are available from the manufacturer.
Remove a friction clip if there is no evidence of damage to the clips. Test the blinds. Sometimes there is an excess of friction that keeps the blinds from operating smoothly, and removing a clip sometimes solves the issue.
Snap the blinds out of the brackets and remove them from the window.
Take the end caps off the bottom rail.
Search for the sections of steel rod, and remove one of them. Replace the blinds and test for sagging.
Remove another section of steel rod if removing one was not helpful, and test again. When the blinds cease to sag, replace the end caps on the bottom rail and snap the blinds back into place in the brackets.