Thanks for your question. Climate models do not incorporate any adjustments explicitly for ENSO in response to climate change, but rather the changes in ENSO emerge from the effects of greenhouse gas warming interacting with the physical laws encoded in the ocean and atmospheric models. In other words, climate models consist of equations that capture physical laws like conservation of energy and mass, and the various weather and climate phenomena like ENSO emerge as a result of the ability to simulate these basic physical processes in our climate system. Although the simulation of ENSO may be factored into the "tuning" of various parameterizations (parameterization is needed for processes that cannot be explicitly simulated by the climate model, mainly because grid cells are too large to simulate smaller-scale processes), climate models do not explicitly target the simulation of ENSO. It's actually reassuring that fairly realistic simulation of ENSO emerges without this explicit goal of simulating ENSO. This site provides a brief overview of climate modeling that may be helpful. 

The effects of increasing greenhouse gases are incorporated into the radiative transfer components of our climate models, and the resulting temperature changes in the atmosphere and ocean affect all sorts of phenomena like ENSO through the interaction of these temperature changes with the physical laws and parameterizations encoded in our atmosphere and ocean models. Therefore, the changes to ENSO again emerge from the basic physics captured by these models, but there are no ad hoc adjustments specifically for ENSO.

We have published several blog posts related to ENSO and climate change. You can find them here, here, here, here, and here. I hope that helps!