How to Modify Live Weather

[N316TS]

Hello,
Unfortunately, you can’t. Here’s an applicable Wishlist topic for what you are trying to do:

Create custom weather from weather presets or live weather

I think it would be nice to select live weather or any other weather preset, then change to the custom weather, with all the settings from live weather or the preset ported over. That way, you can change only a few parameters (for example less wind, less lightning, etc).