Shorter harness lines automatically ensure an upright stance which gives you more lift to go upwind. Sheet in the sail as much as possible. If the front leg bends unintentionally, it helps to place the back foot in front of the strap. Try to tilt (“angle”) the board when you’re going upwind.
To do this, place the feet all the way out on the rail and lean out hanging on the sail. This way you can point upwind to the max.