Stepping down/up center tapped transformer
Your transformer won't be supplying DC voltages, it will supply an AC voltage to another circuit board with a bridge rectifier (square black thing) and two electrolytic capacitors (round blue things), this PCB will have voltages of +35VDC and -35VDC.
While you could connect a buck convertor to reduce the +35VDC to +20VDC, you can't use the same type of convertor for the negative supply, as the buck converter has both negative terminals connected together. You might be able to find a negative supply buck converter, but they would be rare. You could however use an isolated convertor for the negative supply.
If your transformer had two seperate windings, then you could use two rectifier bridges, two capacitors, and two buck converters.
Sometimes just getting another transformer is simpler!
Note. In case you are not aware of it, the rectified DC voltage is 1.4 times the AC voltage of the transformer, so a 20VAC winding produces 28VDC,  a 24VDC winding produces 34VDC, a 14V winding produces 20VDC.