There are several ways to extend an existing loan.
The most obvious is the Extend button on the Circulation screen (but you need to resource!):-
Click the Extend (Ctrl + X) button in the Circulation module
Zap or type the Resource Barcode into the Extend Current Loans screen
(you will need to press <Enter> or click OK if you type it in)
The Extend Loan screen will appear with the Due Date automatically extended to the same date that it would be if you had lent it out today to the borrower
You could further extend the loan by typing in a new Due Date manually or clicking on the "..." button to select a date from a calendar. (Don't forget you can click on the month in the popup calendar to select the month you want from a list!)
The Extension is granted as soon as you press <Enter> or click OK!