You have done the hard work, as you have a variable called grounded, so now you can just adjust the speed when grounded = 1. Set a variable definition where you define 2 variables for speed, say spd_ground = 4 and spd_air = 6. Now in the section where you have the Set Speed block and are setting it to 4, instead change it to an If Variable block, where you ask if grounded is true. If it is true, set Set Speed to spd_ground, else Set Speed to spd_air.