How to switch over a 32 bit to a 64 bit

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

If you use Final cut Pro you may have some preparatory work to do before Catalina.

Macs have handled both 32 bit and 64 bit applications for some years now, and as of the present still do. When Catalina comes out in the autumn it will handle only 64 bit, so that older applications which are 32 bit will simply not run, and it will be up to the makers of such applications to update them. In many cases with older ones no update is likely to be forthcoming and so a replacement will have to be found.

So at the moment, and until you upgrade to Catalina (which I wouldn't be in any great hurry to do) there is nothing you need to, or even can, do.

You ask the developer of the app where you can get an updated version.

While you're at it, you might ask them why they haven't provided one in the 12 years Mac OS has been 64-bit. That answer may give you some idea as to whether you might want to find a replacement app from a developer who is actively working to improve their products.