By default, a licence owner can always release a licence from a site by using the “Disconnect” button on the “Plugins” page in their WordPress dashboard. This is the recommended way.
However, if the licence owner no longer has access to that page (e.g. the site was deleted, or belongs to a client who no longer permits them to have access), then this capability becomes available in their account area on your site (assuming that you have used the relevant shortcode to show them their licences there) 30 days after an update was last checked for – after this time, it’s assumed that the plugin is no longer installed on the site. If you want them to be able to release it before this, then you can use the filter shown below on your site; in this example, they can release a licence at any time:
add_filter('updraftmanager_entitlement_when_can_be_reset', function() { return -1; });