In myBooks, you can convert the Proforma Invoice into Invoice by a click on Transactions – Proforma Invoice, All Proforma Invoice List page opens
Select the Proforma invoice that you want to convert,
Now you can find the Convert to Invoice option above the Filter By Status, Then the selected Proforma Invoice will be converted to Invoice.
Also, Under the Actions tab, next to the print option you can see the convert to invoice symbol. Just click on the symbol to convert it as an Invoice.