A common DIY question: can you just use a C-clamp to push the caliper piston back in for new pads? Often yes - with important exceptions. Here is when a C-clamp works on a caliper.
Fine for straight-in front pistons
Most front caliper pistons push straight in, and a C-clamp (using an old pad or flat plate to spread the load) works to compress them; see caliper compression tool. Go slow and keep the piston square so it does not cock.
Not for twist-in rear pistons
Many rear calipers with an integrated parking brake have pistons that must be screwed in clockwise, not pushed - a C-clamp will damage them; see new pad won't fit. Those need a wind-back tool.
Open the bleeder first
Whether C-clamp or tool, crack the bleeder screw while compressing so you push old fluid out to a bottle instead of back through the ABS - cleaner and easier; see brake service tips.
Do it right
Confirm whether your rear piston pushes or twists before forcing anything. Use a spreader plate, go slow, and manage the fluid. Then fit quality pads and hardware.