I think it was meant to be all other genres already in the files.
That is, if you select ten tracks that have different genres, selecting them all and editing their genre will replace any previous values. It sounds like this is what you want to do though and selecting all the tracks and selecting Edit is the way to do it.
The alternative is to open the Tag Inspector - which is an option available from within the Editor window - and then add a new genre tag. This will add your genre tag to the previous values instead of replacing them and would be a less common thing to do.
Bee excellent to each other...