You can reassign an assigned floating IP to point it at a different Droplet. You can also unassign it from a Droplet entirely.
To reassign or unassign a floating IP from the control panel, click Networking in the main navigation, then click Floating IPs.
Your existing floating IPs and the Droplet they’re pointed at are listed in the table on this page. Open the More menu of the floating IP you want to reassign or unassign:
From the More menu, choose Unassign to unassign the floating IP entirely, or choose Reassign and pick a new Droplet in the Search for a Droplet text box that opens. You can only reassign floating IPs to Droplets in the same datacenter.