Quick question for Asus 520 router

I have to reset my router, because I am not able to login in with current password. Do I reset the router while it is plugged in and press the reset button at the back? Can anyone tell me what lights I should see, to know when I should release the reset button?

2) I am going to flash my router with one of the tomato versions, one of the instructions has the following line ...
9. Wait for two (2) more minutes BEFORE you reset the WL-520GU router.

10. Power cycle your router
What does it mean by reset the router? Does it mean to press the reset button, while plugged in?

3) For future reference once I flash the router will pressing the reset button bring the router back to the old firmware?

Thanks