To do this, first select the account in the account list.

1. Now you can change or delete the tag directly in the transaction's list with a right swipe

2. Open the transaction details for a transaction. Here you can add new tags via the tag symbol in the upper right corner, select an already present tag or delete tags. 

To delete a tag that is assigned to multiple transactions, search for the tag in the search field > tag all results > delete the tag and enter a new tag if you want.