7020 SFF

Bought a used Optiplex 7020 SFF and want to add a faster drive by using the PCIe slot and adapter for M.2..... I cloned the HDD to SSD but now it will not allow me to change the boot disk. Result is that it will only boot from that original drive. If I disconect power and data cable to the HDD it will not seek for a new bootable location. What should I do? I'm afraid to pull the CMOS batt for it could get worse. How can I get password for an old corporate computer?