How to redesign the circuit such that the switching threshold is VDD/2.
Fix the NMOS dimension
Vary the PMOS width until it will give you the same current as the NMOS at |VGS|=VDD/2. The PMOS width is usually 2-3x more than the NMOS due to ratio of hole and electron mobility.


Note that it will only be precise at only one supply voltage, temperature value and one process corner, without taking into account statistical shifts due to mismatch. Also, an inverter has other specs than the threshold voltage. Threshold is just of them. (propagation delay for given drive and load, ride and fall times for a given load and their symmetry, minimum/maximum supply)