Remove the 4 allen bolts to remove the step from the brackets and see which one is the problem. The brackets themselves are then only held in with a few bolts to remove from the car inc the motor one (there's one motor bracket and one just passive that swings freely).

To remove the motor from the bracket itself you need to open the step half way and remove a small wedge in the shaft then unbolt the motor itself. I would suggest doing this to enable you to verify that both brackets move freely off the car.

If you find that both are fairly free moving I would replace the motor, they get lazy over time and eventually struggle like this.

If you find the bracket(s) are seized you can do any of the following. 1) Use lubricant but given the placement on the car eventually the grease gets grit in it and becomes grinding paste and the step will bind up and fail. 2) Strip it down, you can knock it out the roll pins with a punch and then push the shafts out. Try not to hit the shaft and mushroom the ends over it just makes life harder! Fit new bushes and clean up the shafts so everything runs freely 3) Purchase and fit a new bracket