How do I remove this (bimini top mount)?

[jamesk]

How do I remove this (bimini top mount)?
trying to replace with a different type....the screws just spin when I try to unscrew them, not sure they don't have nuts on the other end maybe?
Can't figure out how to access them if that is the case as well.
Any ideas?
Thanks!

 

[zipper]

Yes, there is a lock nut and washer under there.

 

[jamesk]

thanks for that....maybe I will skip trying to replace it.
Last boat (crownline) just had screws so I thought this would be the same.