So how do I update to 64 and will this impair any applications?

Mojave is already 64 bit. You can identify your current 32 bit apps by using the System Report, sort accordingly. When Catalina is released, those 32 bit apps won't function.

Note: MacOS has been 64 bit since the introduction of Lion 10.7.

It’s your existing 32 bit apps that will need updating, not macOS or your Mac. If you have any apps that are mission critical you need to check to see if they are already 64 bit or if they can be updated to 64 bit. That all depends on the developer of those apps. The more responsible developers will have already made their apps compatible with macOS 10.15 Catalina. Older software may have been abandoned by its developer with no hope of an update. Be sure to check before you decide to move to Catalina.

Duplicate.