Who can change the code governing how much of BTC is released? I understand that BTC's will be continually released by the year 2040, when it will reach 21M coins. Actual numbers don't really matter, but I'm interested in who has the power to change this rule. Is this an individual, a "github community"?
It's a bit complicated. Anyone can do it and nobody can...

Let's assume that you want to increase the total number of bitcoins to 42M. To do so you need to:


implement it in code
convince the community that it is a good thing
make a hard fork and hope that miners and users will support you


Something similar but in a smaller scale was happening for the last few years. It was about bitcoin block size, not the amount, but the principle was similar: people were suggesting 2MB blocks, 8MB blocks, variable size blocks and many other things and were trying to convince people that it is necessary, making there own forks and hoping they will become a real bitcoin (Bitcoin Cash, Bitcon ABC, Bitcoin Insert Your Fork Name Here)

As you can see, the block size is still 1MB. The same is with the total amount of bitcoins - you need to convince the majority of the community, then it can become true. But I doubt that anyone could ever do it.