How to flatten Aqua Control + ?

Hello.

I bought Aqua Control + with custom print almost 2 years ago. I used it for like 2 months before it started to curl up on sides. I ignored it at first as I didn’t used that much space, but now it’s getting worse and I wonder what’s the best method to flatten it out? I placed heavy books on it for 2 days, but it didn’t do much. Also what is the best way to clean this pad?
Order #126349
Thank you for response.