I heard that a practice called "cognitive edge conditioning" (basically consistently pushing yourself to the limit in terms of mental activity with progressive overload in mind) is a way to increase one's mental and intellectual limits.
Alternatively, you could double-down on reading educational books along with listening to and/or watching likewise podcasts and YouTube videos to increase one's knowledge stores (taking online-courses helps, too) -- then you can apply all that knowledge you gained via creating something with it; not only does creating something new to help people help you retain all that knowledge, you can also potentially make a livelihood out of it.
So if you want to advance your IQ, there's options for you - I hope this helps, @AlaskaNativeManitou :>