How to read back EPCS64 flash data by FPGA after its configuration

You can use Examine option in Quartus programmer to read back the flash data.

Open Programmer --> Add Device

Choose your FPGA device and click Okay. (For my case I use cyclone 10 LP 10CL08Y)

Right Click on your device --> choose "Attach Flash Device"

Choose your Flash device and click Okay. (For my case I use EPCQ64)

Finally, Tick Examine and click Start