for those who are interested in how to edit players' potentials, here's a way to do it since you can't edit it manually:
Select the player you want to have a better potential and edit his ratings to the maximum ratings you would like to have him he can go up to in his career. This includes all attributes you would like this player to be able to get better at (awareness, catching, route running etc.).
When you're finished with your editing and have your player at the very best he can become in your idea, save his ratings and exit the player editor. You will now see (if in franchise) that his potential has increased.
Now go back into the editor and set his ratings back to his original ones before you edited the player (important: you should know which ratings you changed and what his ratings were before). When you have him back at his original numbers, save and exit the editor.
You now have your player at his original ratings but with a higher potential grade. Now he will be able to progress to the maximum you chose for him.