OpenSolar offers two different softwares to calculate system output.
The first is PVWatts6 (legacy).
The second is System Advisor Model (SAM).
You can learn more about both of these calculators here.
Changing between Legacy and SAM is simple. If you would like to change the modelling software for a specific project, head into the studio zone, scroll down to the bottom of the left sidebar and click the OpenSolar 3D toggle to ON. When switched on, your project will be modeled with SAM. When switched off, your project will be modeled with Legacy.
If you would like to make one of these modelling softwares the default (so that every new project created uses this), head into the control zone > design & hardware > setbacks & design settings > edit/create your default > simulation settings > and select from the OpenSolar 3D dropdown.