If you haven't done them before...
1. Lift car up.
2. Wheel off.
3. Drift the pins out.
4. Remove outboard pad.
5. Push outboard piston in.
6. Scrape all the muck out of the caliper.
7. Wipe some copper slip along the top and bottom edge of the new pad backing and either side of the shim, if present.
8. Fit new pad.
9. Repeat steps 4-8 on inboard pad
10. Refit pins/spring.
11. Refit wheel.
12. Drop car and tighten wheel nuts to 110Nm.
Repeat for opposite side
Sounds long winded, but takes less time to do than to type
The reason is simple, if you remove both pads, then as you push one piston in, it will simply push the other one out, literally... Both messy and annoying.
By doing one pad at a time gives the second piston summat to push against, thereby keeping it in place