Load up your save file in the editor.
Browse Gene tab. load up your monster and the monster's genes will be indicated in the right side top to bottom. consider them as genes 1 to 9 (top to bottom). in your switch's MHS2 game the genes are arranged in a 3x3 colum and rows. gene no. 1 is the top left, three is the top right. 5 is the center, 7 is bottom left, etc.
you can now reassign genes or open gene slots as you wish. see the googlesheet i mentioned a couple of posts back for reference as the editor only allows you to edit the ID, example, 277 which when you look in at the googlesheet is for the One-shot attack gene (M)