To change or delete tags on a transaction, start by selecting the account from the account list. Afterwards you have two options:
In the transaction list, you can tag the desired transaction directly with a right swipe or delete existing tags.
Open the transaction details in the transaction list. Here, you can use the tag icon at the top right to add new tags, select a pre-set tag, or delete existing tags.
To delete a tag that has been assigned to multiple transactions, it's best to search for the desired tag (#tag) in the search field > Tag all results > delete the tag and assign a new one if needed.