Hello and Welcome to justanswer..

You need a special tool to get the piston back into the caliper to enable the fitting of the new pads -

Most good car tool / accessory shops will have these or search on line if you have time for that -

You are needing a brake piston wind back tool - The tool applies pressure to the piston and you screw the piston in..

You may be able to do it with "other" tools - grips , long nose pliers etc sometimes works... the piston must be screwed in though - do not force it, you will damage the brake system inside..

Im very pleased to have helped you -

Please click the Accept button for me now - Thank You ...and if you do need / wish to get back to me after - you can do so just by coming back to this post and use the Reply button - your reply will then come straight back to me .. and Im always happy to help further....

.Kind Rgds to you - MIKE.