That would not work. The gen 9 CPU is a socket 1151 on the motherboard where as the 12th gen is a socket 1700. They are in no way compatible.

@MarcDS97 - What's the goal here? 

What are the other specs for this PC, eg: RAM, GPU, HDD or SSD...?

A 9th Gen i5.9xxx or i7.9xxx would fit in the CPU socket on this motherboard.

JOcean is correct.  12th gen intel cpu needs a completely different motherboard with different cpu socket as well as chipset.  Dell sells that in the newest Inspiron, optiplex, and XPS desktop models.  The 3670 cpu socket can only support up to 9th gen cpu, although there is room to upgrade from your i3 to i7/9 cpu of same gen as already mentioned.  Still at this point since pc tech is advancing so rapidly every 6 months, the investment in an i5/7 9th gen cpu that is already getting outdated has doubtful future proof.  The exception to this is if you can get hold of such cpu dirt cheap.  Unfortunately most likely not at this point (old cpu does not just drop in price precipitously).   A quick check i7-9700T $180, i5-9600T $110, i5-12400 $120.
I would not upgrade cpu in 3670. If you really want to upgrade, sell off the 3670 and build your own 12th gen desktop, or a newer Dell.