Also, for people that dont have a brake rewind tool, you can use any kind of clamp thats big enough, I used a silicone tube gun, without the silicone, and it pushed the piston down easy enough
Failing that you can try and use brute force to push the piston back, this can work but its an absolute ba*tard to do it, normally you need to put something like a bit of wood across and pull it towards you, previous experience can tell you that this involved a lot of swearing and grunting, but can work if you've no other option and got all the way to the reassembly stage without realizing you had no clamp or brake rewind tool.
And also, for people thinking of changing rear disks at the same time, you need to remove the whole hub mount, so probably better having a garage do it because theres a lot more work involved to get the whole caliper removed from the disk, all because the allen key bolt at the top is innaccessible without removing the hub... Which has 4 annoying bolts that arent especially easy to reach, so yeah, rear disks is more likely a garage job unless you're up for the hassle it was for me, took about 3 hours for me to get the disks changed, mainly because of how long it took to figure out how to remove it in the first place. Front disks should be easy enough though