PopSockets are super sticky so it's not always easy to do.
The best way to remove it is to flatten it and peel up slowly from one side of the platform (the sticky part), don't pull up with your PopSocket expanded or it might Pop out of the base.
If you're having trouble with that, try a bit of dental floss! Just slide it under your PopSocket platform to peel it away. REMEMBER, your PopSocket shouldn't be exposed to air for more that 15 minutes or the gel will dry out. Stick it to a window if you haven't figured out where to place it yet.