You can use a potentiometer which is a three-terminal resistor to vary voltage or resistance:

It has a resistive track connected to two end terminals and a wiper that slides over the track
When you turn or slide the knob, the wiper moves, changing its position along the resistive path
This divides the voltage between the two ends, giving a variable output voltage at the wiper
The closer the wiper is to one end, the lower the resistance from that end and the higher the voltage at the output.