Go to Sales in the top menu bar and select the offer you want to edit. In the right column you can see the progress of the offer and the status in the upper right corner. As soon as you or your customer has set the status to "accepted", a new button "Create invoice" will appear in the same place and you can convert the offer into an invoice.
Good to know: You can always manually change the status of an invoice from "open" to "paid". To save yourself this effort, you can also work with BESR.