Ramp/Pulse VCO - how to change the frequency?
You have to change C not the R in the pot. In this case its 50pF, you either use a variable capacitor or varactor (to make it voltage controlled). In the last equation Fo it states that freq is determined by the main freq determining element C.

R changes amplitude not frequency this just an oscillator circuit, making C a varactor will convert this to a VCO.