When creating a promo, select 1-3 genres that fit the style of music you are promoting.

Hover over Releases and click on + New Release

In the popup window, select 1-3 genres from the drop-down list before clicking Create Promo

To update a promo's genres, hover over Releases and click My Releases

Select the respective promo in the left column and click on Edit

Scroll down to the Genres row and make your new selections. Click Save.