You need to figure out the IP address of the printer. It'll look something like: 192.168.1.xxx, where the xxx is some 1-3 digit number. If you know how to get it, you can type that number into a web browser and access the web server of the printer. When you get to the login page, just log in. By default a username and password is not needed.
Then click "Settings/Registration" at the far right of the page.
Next click "Timer Settings" in the left menu.
Then click "Edit" on the far right.
You shoud see the options for changing the sleep settings there.