Assuming the pistons haven't gone beyond their normal travel, just whack a plastic tyre lever in the gap and push fairly hard. When you reinstall the wheel it will take a few pumps on the lever to get the pistons back out to where they're meant to be, but there should be no harm done.
Assuming the pistons haven't gone beyond their normal travel, just whack a plastic tyre lever in the gap and push fairly hard. When you reinstall the wheel it will take a few pumps on the lever to get the pistons back out to where they're meant to be, but there should be no harm done.