Do I just undue the allen key bolts to change the brake pads?

I doubt it, it seems a little too easy. I would check a repair manual before I attempt anything.

not too sure what size they would be but usually they are 7mm allen key bolts on the back of the brake caliper, yes undo them and them push back the piston in the caliper by means of leverage or large water pump pliers, then clean the carrier in which the brake pads sit on then install new brake pads in reverse order

First, "undo" only one of the bolts that hold the calipers on to the frame. Then flip the entire caliper off the disc - it should still be held on by one bolt. Change the pads - you might as well change both front or both rear at once, since L & R pads wear at the same rate. Then compress the pistons - no fancy water pump tools needed here - just a C-clamp from the local hardware store for a few bucks. Flip the caliper back on, replace the bolt. Start the car, pump brakes a few times. Congratulations, you've saved yourself a few hundred bucks! Changing the brakes is probably the easiest maintenance job to do yourself - and less messy than changing the oil.

As Jonathon answered 10-years ago, I bought a set of allen wrenches that included 7mm for my 1999 Volkswagen Golf 4 Dr GL Hatchback and they worked. Calipers came off easily. Most allen tool sets I found had 6mm and 8mm, but no 7mm. Thank you O'Reilly Auto Parts for being there when I need you and the Veteran discount.