firstly, be careful!
the piston needs to slide back straight into the caliper. if you try and force it at an angle it might seize. and if you push even harder you might manage to score the bore and damage it permanently.
firstly....clean everything really well with brake cleaner. this will remove as much old brake dust as possible. even scrub lightly around the piston with a nylon brush.
then - before pushing it back, make sure your master cylinder is almost empty. If it is not it will overflow when you push the pistons back.
then, you need to push them back as straight as possible. use something long and wide to lever them downwards like a large spanner or thin hammer. take it slowly and change from side to side to make sure they go evenly. a g clamp is even better it will push them back straight.