To replace Teg you should run a query UPDATE. First you need to go to K2-> Tags and find out under what id is required tags. Next in phpMyadmin we insert sql query code like this:

-- If we want to replace the tag id 6 wherever there is tag with id 12, the request will be so
UPDATE `you_prefix_k2_tags_xref` SET `tagID` = '6' WHERE `you_prefix_k2_tags_xref`.`tagID` =12;