For the integrity of your data, you cannot edit an existing tax. Remember the tax you want to edit has probably already been used in some transactions. To change it, if already used, is an action that may alter the consistency of your data. If Nutcache prevents you from editing it, it’s precisely because you need to keep your data intact.
However, Nutcache offers you the option to create a new tax or to use multiple taxes at the same time, according to your needs.