That depends on the situation.
We have a resistor of value R, that is connected across a potential difference V. Then a current of intensity I flows in that, where I = V / R. The power (instantaneous) dissipated as heat is = P = I² R or V² / R.
Then heat generated over time T is H = P T = I² R T = V² T / R
Once we understand this much, we need to know if the resistance is connected across a current source which has a fixed current or across a voltage source like a battery. In this case the voltage is constant.
In case the voltage V across R, is constant, then H is inversely proportional to R. In this case the current varies inversely with R and hence we have to substitute I = V/R...
In case the current passing through R, is kept constant, then voltage across it varies directly as R varies, then we use the formula : H = I² R T. Then H is directly proportional to R.