The first thing to concentrate on is your stroke rate. You want to paddle with a high stroke rate to make good progress into the wind, this can be achieved by shortening the length of your paddle stroke and turning over as many strokes as possible. Think of your stroke rate as gears on a bike and the wind as a hill, in order to get over a hill we lower the gear on our bike just like we increase are stroke rate on a paddle board.
Another thing to focus on is your hand position on the shaft of your paddle, by moving your bottom hand down the shaft you will be able to achieve more leverage and torque in each stroke. Moving your hand down the shaft will also help you achieve a shorter paddle stroke needed for a higher stroke rate.
Make sure you get as low to the board as possible in order to decrease the amount of wind resistance your body is creating. If the wind is very strong you may want to try a chock stroke. This is where you hold the paddle with 2 hands on the shaft of your paddle allowing you to get even lower to the board.
Remember that the nose of your board is also going to create wind resistance. When the nose of your board sticks out of the water wind hitting the front of your board will slow it down so you should try and get the nose as close to the water as possible. To do this you will have to move your feet up closer to the nose, it is best to do this gradually and be careful not to bury the nose of the board in the water.
When doing the recovery phase of your stroke make sure to feather the blade of your paddle by twisting your wrist inwards, ensuring your paddle makes the least resistance as possible during the recovery.
Remember when planing your SUP session it is always better to start paddling upwind rather than finishing finishing with it.