This is a really good video that covers a lot of points that will help with getting upwind in the lighter stuff.
The advice about riding as fast as you can is actually not true - you need to find the right balance between board speed and available power. Going as fast as you can does give you more apparent wind, but apparent wind also changes the
direction of where the wind appears to be coming from, and that always shifts to make you go more
downwind compared to the base wind direction. The guy in the video explains this a lot better than I can, but it is something I have seen a lot of learners struggle with.
Also, the other tip above about really sineing the kite is correct to a point - but sineing also drops the kite back further in the window - which pulls you further down wind. Again, it's about finding the balance between speed, power and angle... which is where the tip about "practice, practice, practice...' is spot on. You want to sine the kite enough to keep it where it is giving you the right amount of power to maintain your speed - any more than that and it will be starting to pull you downwind (and beginners generally react to that by edging harder against the kite and killing its power).
You don't want to be locked into an upwind angle either. In more wind, you can lock yourself in to an angle and just keep edging in that direction. In the lighter stuff, if you can feel the kite starting to lose power or choke, bear off a bit to let it breathe (flatten the board and go slightly with the kite), then when it recovers, you can start pushing more upwind again.
If really you want to ride in the lighter stuff, the biggest bang-for-your-buck is a light-wind board, followed by a bigger kite. In 15-18 knots and 75kg though, you should be able to go upwind decently on the gear you have with the right technique, although the waves will be sapping your power a bit.
Sorry, that was meant to be short... Just watch the video...