@SamStrangward along with @dcdenu4’s pointer to the Scenario Generator, I’ll chime in a little more about creating scenarios. How they are created varies widely, depending on what kind of data you’re starting with, what kind of scenario it is, and what kind of information you’re incorporating into the scenario to change it from baseline conditions. So it’s really hard to provide generic guidance.
If you already have spatial data about where changes occur (in a shapefile or raster, for example), you can probably just use GIS techniques to change the appropriate pixels in the baseline landcover map, then update the biophysical table if needed. If your change information is not yet spatial, you’ll probably need to make it spatial by reclassifying certain LULC types based on how they’re likely to change, or whatever is appropriate for visualizing the elements of your scenario. Perhaps only certain areas will change, like a buffer around towns or rivers, in which case you’ll need to make these buffered features, and use them to change the pixels in the baseline landcover map. There are many possibilities for what’s needed to realize your scenario, thus many possible GIS techniques.
It is also possible that your scenarios only change the biophysical table, not the LULC map. For example, if you want to simply age all of your forest by 20 years, you can provide different carbon pool information without needing to alter the LULC map.