I wonder if there is a possibility to edit the source image of the patterns from the preset manager in Photoshop. On right click on a pattern the only options are Delete Pattern and Rename Pattern.
Once they're created they're stuck like that forever. Like when you get that stupid "Macarena" song stuck in your head at a wedding.
What you can do, however, is create a copy of the pattern, edit it, and add it as a second version. Create an empty square canvas (you'll have to adjust the size depending on the pattern) and fill it with the pattern (Shift + F5, select "Pattern"). Make your edits and save the file (in case you don't get it 100% correct). Select the entire canvas (cmd + a), go to the "Edit" menu, and select "Define Pattern..."