The undo option for "Thank you", is disabled by default. If you want to enable that on your website you can execute this query in your UNA's database.

UPDATE `sys_objects_vote` SET `IsUndo`=1

Then clear the cache via Stuido->Dasboard panel. If you need to update likes only for the specific module - then the query should have a look like 

UPDATE `sys_objects_vote` SET `IsUndo`=1 WHERE `Name`='[name of the module]'  (like bx_albums or bx_events).

source: https://una.io/page/view-discussion?id=996