At first select the account in your list of accounts.
1. If you move your courser over the transactions you will see a blue button 'Add Tags'. Click on this button and you can either delete existing tags or add more tags.
If you want to delete a tag, that is already assigned to more transactions, pleas search for the tag in the search field. Then choose 'tag all results' > delete the tag und add a new one.
2. Open the transaction details for a single transaction. Here you can also add new tags or delete tags by tapping on the tag-symbol in the toolbar.