How to disassociate floating IP?
If you no longer need public IP address you may disassociate it from your VM. Click "Dissasociate Floating IP" from the dropdown menu:
Floating IP’s as any other Openstack resources have their cost when kept reserved and not used.
If you don't want to keep your Floating IP’s reserved for your project you may release them to the Openstack pool for other users which will also reduce your project costs.
Go to Project → Network → Floating IPs
For the address that is not in use, the "Release Floating IP" option will be available. Click it to release the IP address.