Depends on which one.

The reason it's relatively easy to chip an OBD1 Honda ECU is that it has a separate memory chip which stores all the maps and such that the ECU uses and that chip can simply be replaced. Their non-OBD ECUs are like this as well. In addition there are a couple companies which modify the non-OBD and OBD1 ECUs to be programmable by the user: ZDyne and Hondata.

The OBD2 D-, B- and H-series ECUs have a memory chip which is integrated with the processor. There's no way to replace the chip unless you also replace the processor, and that's expensive enough such that no chip companies have bothered to try.

The OBD2 K-series ECUs use flash memory which can be reprogrammed, and Hondata offers this service. Hondata also modifies K-series ECUs to be fully programmable.

Essentially if you want to chip an OBD2 D-, B- or H-series car you need to replace the ECU entirely. You can do that either with an AEM EMS standalone that will plug into the stock harness, or you can get an OBD1 conversion harness that will allow you to run a chipped/modded/whatever OBD1 ECU.

As always, you don't need to chip a Honda four-banger ECU unless the motor has been modified beyond the standard bolt ons.