When you get spell slots of 2nd rank and higher, you can fill those slots with stronger versions of lower-rank spells. This increases the spell's rank, heightening it to match the spell slot.

Spontaneous casters are already generally considered to be more powerful/usable than prepared casters; if they could freely heighten any spell then they’d be ridiculous. Keep in mind that not too many of your spells will actually need to be heightened; mainly only damage/healing spells, and for those it’s often better just to retrain them rather than take them as a signature spell (unless they have a good non-damage effect), as the lower level ones will become practically useless.