Xbox 360 Change theme?

Can someone tell me how to change the themes that you already have downloaded.

I know you can download and change from there, but how do you just click on your theme and change it for already downloaded ones.Xbox 360 Change theme?
Go to ';My Xbox'; and sign-in if you aren't already signed in.

Go to ';My Xbox'; and select your avatar (The card with your gamertag info on it)

Press A to select it

Press Down 3 times to select Change Theme.

Select that

Here is the list of all your themes you have (that your account can use).

Select the one you want and press A to change the theme.Xbox 360 Change theme?
Go to the dashboard. Go to My Xbox and move over to the right where your avatar is. Click on that and it will say change theme.