Thank you very much for your question.
The pistons have to be pushed and screwed back into the calipers. There is a special tool to do this job which you can buy from Halfords or another accessory shop, but you may be able to do this with a pair of pipe pliers or similar.
Take care not to damage the rubber seals whilst screwing the pistons back in. To see the tool, please see the following link.
I hope this answers your question.