If your upgrade includes a new psu that has molex 4 pin connectors then you may replace stock back exhaust fan with new 92 mm fan, and add a new 80 or 92 mm intake fan and mount it to front grill using 4 grommets.  Stock motherboard has only one sys fan header which only works with Dell OEM fan.  If not concerned for noise or lack of led fan, no need to replace stock back fan. If you intend to remove hdd cage, more room for a front fan. more in depth mod involves cutting chassis to mount multiple 120mm fans should you feel ambitious.  It was done by a Reddit user.
So, I now have a refurbed Optiplex 9020 i7 MT 4770 on the way, & while I narrowed down everything I plan on getting to upgrade it, the one thing I haven't been able to get any specific advice on is replacing or adding extra cooling fans (not the main cooler), something that should be key to anyone's pc upgrade, if not expensive. What would be recommended for this largest tower unit in terms of the maximum size allowed, available placement area(s) (one by the PSU), the required number of amount of modular pins, & how many can be installed inside the case, I'd like at least 2.
Extra Cooling Fans For Optiplex 9020 i7 MT 4770? Specs, Etc.