How do we adjust a FET's threshold voltage?

Do you mean the voltage at which the FET is ``on''? The behavior of the FET as a function of $V_{gs}$ is a property of the FET (and depends on whether it's a JFET, an e-MOSFET or a d-MOSFET). To decide how to choose $V_{dd}$ and the ``pull-down'' resistor $R_d$ in a FET switch, you find a solution like that described in Eggleston 5.3, using the $I_d$ vs $V_{ds}$ characteristic of the FET, and select voltages for which the FET is in the plateau region with small or large $I_d$ when $V_{gs}$ is high or low.