upgrade firmware on S4048ON switch

Hi,

I am in need of upgrading firmware on S4048ON. The current firmware version of the old switch is 9.11 (2.1). I want to upgrade to version 9.13 (0.1). I have received a new switch which is having 9.13 (0.1) version. To connect stack I need both switches to have the same version  which is 9.13 (0.1). I would appreciate if you could send me the easiest upgrade procedure.

NOTE: Kindly let me know whether the stack can work without  upgrading the old switch.