If you mean how is it done, then the most common way is to have switches and different lengths of microstrip lines to switch in 22.5, 45, 90 and 180 degree lengths. typically that's for phased array antennas. At lower frequencies (under 500 Mhz), instead of switching in differenct line lengths, groups of LC combo's are used (inductor/capacitor) to acquire the phase shifting.
Otherwise, some of the newer technologies have a long line of variable capacitors that are biased and the electrical length changes with the bias voltage, Agile Materials makes this new product.
Mini-Circuits and Hittite corporation have some of the GaAs technology phase shifters.
kch.