Before you can do this you need to remove the panels on the machine. Before doing this, make sure your machine is unplugged from its power source, any direct plumbing lines are closed and the machine is fully cooled off.
First, remove the capillary tube fitting that connects the brew gauge to the pump using an adjustable wrench.
Second, you'll want to move the motor out of the way to make space to work in. Do that by removing the two top nuts shown here.
If you need the whole pump to move, you can unthread the bottom bolts of the pump shown here to make the entire pump and motor assembly movable.
At this point the motor and pump can be lifted out of it's seating and shifted to the left or right to access the brew gauge bracket easier.
Now, use the long Phillips screwdriver to loosen the gauge bracket. Remove the bracket holding the gauge in place, pull the gauge out through the front of the machine and insert the new gauge in through the hole.