In the repair kit you will find a black plastic valve spanner- this is for tightening or removing the valve. The below image shows what a similar valve looks like when out of the board. The left hand piece is built inside the board and the right hand one screws into it via a circular hole in the skin of the board, this traps the board skin layer between the top of the inner basket and the underside of the top part.
Heat expansion during use can cause the valves to loosen, this is quite common. In fact there should have been a red tag on the valve informing you about this when the board was new out of the packaging.
The valves are designed to be removable to allow replacement so are screwed together in place rather than glued. To tighten the valve and improve the seal, remove the dust cover and insert the crown shaped end of the spanner into the valve and carefully turn clockwise.
The only trick with tightening up may be stopping the inner valve piece, located inside the board, turning as you tighten the outer part with the spanner, preventing the assembly from fully tightening.
To tackle this, with the board fully deflated, kneel either side of the valve to hold it down or hold the inner part of the valve through the skin from below whilst you turn the outer part with the valve spanner. It can be helpful to have two people doing this together.
If you’re lucky you may even just be able to apply enough pressure without holding the bottom part of the valve to stop the leak you’re experiencing. However if the whole valve assembly just spins when you tighten it you will need to get some pressure on the underside.
Do be careful not to over tighten and damage the valve.
If after doing this you reinflate and find that you still have a leak, unscrew and remove the top part completely and check there is no dust or dirt inside the valve that is causing it not to close up completely. Take care when doing this however not to lose the inner part of the valve inside the board!