You may use the 'Action' drop-down available in the last column to Edit an Invoice.
This option does not appear when you have already allocated a payment to the Invoice.
Please follow the below steps while editing an invoice whose status is paid.
Step 1: You need to un-allocate the payment by clicking on Supplier Name. Bookkeeping >> Purchase >> Purchases
Step 2: You need to select the Unallocated amount in the dropdown box in respect of the related purchases.
Step 3: Again, you need to go to Purchases and select edit option in the Action drop-down box.