How do you delete (or edit) custom lead source channels?

How do you delete (or edit) custom lead source channels?

I've created two lead source channels that appear in the "Lead Source" field's drop-down menu (screen-1414).

Unfortunately, they don't appear they way I want them to so I want to either edit them or delete them and add new versions. 

I can't see where you can do this. Can anybody point me in the right direction?

Thanks

Steve