How do we update from 32 to 64...

This is a generic warning only. You have to do nothing, 32-bit apps continue to work on both High Sierra & Mojave.

In the future release of macOS 10.15 Catalina, applications will have to be 64-bit. App developers will issue updated apps if they want their apps to work on these future releases.

32-bit app compatibility with macOS High Sierra 10.13 ... - Apple Support

https://support.apple.com/en-us/HT208436

Your computer and operating system has been 64 bit for over a decade. It currently runs both 32 and 64 bit apps. With the next release of the operating system only 64 bit apps will run on your computer. So you have two choices. Do not upgrade your operating to the next one after Mojave, or update your non-64 bit third party apps to 64 bit. You upgrade the third party apps to 64 bit by contacting their respective developers for the upgrades.

64-bit will not take effect until you install Catalina. All Apple default apps are already 64-bit. You should be notified about any third party apps that require an update. Then you can check with the developer for updates.