Precondition: you didn't customize, or you have been following the advised way to customize and edited only the skin's report configs in weewx.conf, and have nothing else changed (like templates that come with the skin, the skin's skin.conf, ...):
- Install the new version, you won't loose any actual settings
If you don't meet the precondition above, things may get more complicated, but there are good news: for customizing whole pages, 4.1 and above lets you do this without having to edit anything that comes with the installer. For your next update, all such customization, if done the recommended way, remains untouched.