There are a number of predefined genres, the rest are those entered in the tags of your files. Open the list/album list view with column browser and turn on the Genres column. Find a genre you want to remove or merge with another. Click on it to show all the files with that genre. Select all & Get info. Type in the updated value for the Genre. Click OK. Repeat until finished....
If you want to edit the list of available genres still left after clearing any that are stored in your tags, see this article.