Intelligence is partly heritable, but there are ways to improve it in a general capacity. The best way to improve your working memory is to simply learn a lot. Learning creates chunks, which allows you to deal with more complex ideas.
The downside of learning is that it tends to be narrow, while intelligence is general.

Intelligence is different from knowledge

Intelligence is likely associated with a better generalised working memory.
A working memory is the ability to hold multiple ideas in your head at the same time. Those who are smarter have a bigger capacity to hold multiple ideas.

Being smart changes how you learn things

Some subjects feel like they require more intelligence. If understanding an idea requires you to take different ideas into account, it may be more challenging to keep track with a lower working memory. e.g. mathematical proofs.
Conversely, if a subject requires a large volume of memory, but each fact or idea is separate, it may require lots of practice and less working memory. E.g. history or law.

Why some subject are harder to learn

Any subject can be learned with some practice and patience. Some subjects may appear harder because of the speed at which they are taught. 
To fix this problem, you may need to put in more work. If you can't see everything at once, it just means you need to practice more of the pieces, so your mind does not have to juggle so many ideas simultaneously.

You can learn anything

Because we can increase our working memory within a subject, we can create new chunks and master very complicated ideas. The perceived difficulty is often the way subjects are taught. Only the smarter students will remain, the higher you go in maths or science. This leads teachers to skip over "trivial" steps, leading you feeling like you can't master it.
Solutions are for teachers to slow down the class or for the student to slow down the class using, for example, the Feynman Technique.

The problem with how a subject is taught

Are some subjects just so hard that you need to be really smart to learn them? If not, why does it often feel that way. Here's the science behind why.

Intelligence is different from knowledge

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