Only sales orders that are finalised can be invoiced.
On the right hand side, there are buttons to create a "manual invoice" or "invoice all".
Manually create an invoice
Using the manual create option, you can select parts of the sales order to invoice e.g. some line items only or lesser quantity etc.
Invoice all automatically creates an invoice with all the uninvoiced items in this sales order.
If you wish to edit the invoice you created, simply click into the invoice under the Invoices section. Click on 'Edit' to make any changes to the invoice.
If you wish to edit an invoice that has been paid, learn more from this article.