You can add approved time entries to existing invoices, a process called Progress Billing. This is done through the Time Entries > Batch Update > Invoice Number option. You need to be in the Show: Project mode in the Time Entries screen. However, the invoice amount will not increase as a result of the added billable time entries because the total value of services is already set. The time entries will be written down to equate to the total service amount of the invoice. That is why we recommend linking time entries to existing invoices in this manner only when you have pre-billed a fixed project. If it is an hourly invoice, then you should delete the draft invoice and recreate it.