How can I edit the transaction categories, as they are mostly recorded wrongly and giving an incorrect VAT rate ?

Hi skylava-outlook-, thanks for joining the Community - you can open any transaction to edit this and change the product/service item or category selected on the drop-down. The category used for products/services (the income account) is selected during the set up, and can be changed by navigating to Sales > Products & services > Select Edit beside the item > See income account field. If you need to create any new categories, you can see how-to here. 🙂