Depends if you put in a pre-order for the alpha stage amount ($50usd), beta amount ($35usd) or for the basic level of preorder($15usd). If you paid for alpha tier, then you should email Lars at email@example.com or PM @larsiusprime on here.
There is a way to increase your pre-order, as well.
from the FAQ page
If I preorder now, can I upgrade to a higher tier later?
Maybe. I've been talking to Humble Store about this, and they say it's technically possible but a bit labor intensive to do the upgrades by hand. If there's enough demand for this, I will try to look into an automated solution with them.
In the meantime, just contact us directly and we'll make sure you're taken care of.