Obtain the UUID of the desired flavor
openstack flavor list
Resize the server:
openstack server resize --flavor <flavor_UUID> <instance_name_or_UUID>
At this point, Nova will perform a graceful shutdown of the specified instance and allow 60 seconds for it to complete.
Check the status of the resize operation by running
openstack server show <UUID/name>
Confirm the resize
openstack server migration confirm <instance_name_or_UUID>
If there are any issues, the operation can be reverted by executing:
openstack server migration revert <instance_name_or_UUID>