Excelsior

I have a bow i'm leveling.

According to the wiki

"Upon gaining a level crafted weapons will gain 3 points which you can spend on upgrades. Non-crafted weapons will yield 5 points per level."

How do you upgrade it?
It doesn't say how in the wiki

I've so far leveled my bow up to 11 and haven't upgraded it any.

Single click the weapon and select Information from the menu, that is the user interface to spend the points.