There is a Github project [1] which has detailed instructions. The ancient i5-3570 only allowed 2GB ReBAR, BTW. GPU-Z says ReBAR / 4G is activated and working, Intel Arc Driver does not see it but seems to use it. Some part of the BIOS had to be manually fixed, AFAIR.

The PC was given for free, the CPU €11 yet overall I wouldn't recommend the process just for the result. It's only little benefit, if at all, though fun. On that occasion I also added some NVMe driver which works well, demonstrated for the similar Dell Optiplex [2][3]..

[1] https://github.com/xcuri0/rebaruefi

[2] https://www.tachytelic.net/2021/12/dell-optiplex-7010-pcie-n...

[3] https://github.com/jrdoughty/Dell-7010-rebar-guide

Edit: some wording