What happens to lava as its temperature increases?

I will assume you mean liquid lava, that is, magma that has been expelled by a volcano, and is flowing downhill, until it cools and solidifies as lava rock. Liquids typically have a generally inverse relationship between viscosity (resistance to flow) and temperature. That is, as the temperature increases, viscosity generally decreases (i.e., the lava gets “thinner” and “runnier”), as Gopismhas said. However, generally, in nature, lava doesn’t increase in temperature, but rather cools as it is expelled and flows downhill, and thus it is getting more and more viscous…until it solidifies.