If you have one of the OptiPlex 7020 mini tower, you can try a 24pin to 8pin adapter which will allow to use a standard ATX power supply of your choice. Otherwise, a Dell part T1M43 or 7VK45 power supply will fit yours without adapter.  But they are 365w only.
 My PCU is 270W and GPU is R470 4GB 256bit DDR5 and I want to put 500W-600W PCU is it possible?
I have a Dell Inc 0F5C5X motherboard and I want new PCU