Hi,

Usually it is recommended to upgrade the BIOS version incrementally. 1.5 to 2.5 is a big jump. Can you check in iDRAC's job queue on the BIOS upgrade progress since you mentioned it's been 3 hours. Is it at 10%? Can you try running this command using RACADM:

racadm -r 192.168.1.1 -u root -p abc123 recover BIOS.Setup.1-1

Reference on how to use RACADM: https://www.dell.com/support/contents/en-us/videos/videoplayer/tutorial-on-idrac-racadm-command-line/1706695616981987241#:~:text=Once%20here%2C%20enter%20%E2%80%9CiDRAC9%E2%80%9D,to%20type%20is%20%E2%80%9Cgetsysinfo%E2%80%9D.

If it doesn't work, you can proceed to try using clear NVRAM method.