You will need to log into your account on Slingbox.com and you will see your directory in the upper left corner. Hover over the units in the directory and a drop down will show to the right with a delete option. Choose the unit you want to remove from you account and click the "x" next to it. Your old Slingbox is now removed from your account.
Hope this helps!!
I'm sorry you are having difficulty locating the directory!! Here is a support article that will give you a couple of different screen shots of what you will be looking for.
Hope this helps you out!!
My original Slingbox M1 had a defect. Half the screen on the TV had a red tint. It goes away with resetting by unplugging power, but it's annoying. I comes back after a few days.
Anyway, I got an exchange unit with the retailer. Now I have two Slingbox M1 in my directory. How do I delete the old one?