To delete existing transaction tags, first go to MoneyWiz Settings by clicking on the gear icon in the menu. Once you're in Settings view, click on Tags section.
To delete a tag, swipe left on it and confirm by clicking Delete button. MoneyWiz will ask you if you are sure.
If you proceed, the tag will be deleted and removed from any transactions that might be using it. MoneyWiz does not check whether the tag is used or not so please check this yourself before deleting the tag (for example by generating a tag report for all time period).