If you've decided to change the name of the payee in MoneyWiz, you don't need to go through each individual transaction to change the payee name there. You can just change the name in one place and all transactions will update automatically.
To create new transaction categories, first go to MoneyWiz Settings by clicking on the gear icon in the menu. Once you're in Settings view, click on Payees section.
To modify a payee just click on it and change the name.