Can't remove fixed IP from OpenStack

My friend, you should delete your useless IP in port, in OpenStack documentation we see this note:

If attempting to allocate an IP that has a port associated with it already, ensure the port is deleted beforehand.

and:

If the port is active, remove the IP from the instance with which the port is associated, or delete the instance altogether.

To find your way out from this issue please work with this description:

1. Find your port and instance data in these commands:

   openstack server list
   openstack port list | grep YOUR_OLD_INSTANCE_IP
   openstack network list
   

2. Delete IP assigned to your port of instance and make this instance without IP:

   openstack port unset --fixed-ip subnet_id='SUBNET_ID',ip_address='YOUR_OLD_INSTANCE_IP'
   

3. Make sure that your instance has not any IP:

   openstack server list
   

4. Now you can assign your new IP:

   openstack server add fixed ip --fixed-ip-address NEW_IP INSTANCE_ID NETWORK_ID
   

Congratulations; now your instance has new IP. Now you must restart your server for find new IP from DHCP or restart network services -- in ubuntu server run netplan apply.

Tanks for your question