What 64 bit software can i use on my MAC instead of macOS High Sierra 10.13.4?

Any version of the Mac OS from Mountain Lion, 10.8.x and forward are 64 bit. Use whichever one you want. Though you are restricted to how far back you can go. You cannot load an OS that is older than what your Mac shipped with.

Your Mac and its operating system are 64 bit and have been for over 10 years. You are likely refering to the 32 bit warning message. This message refers to third party 32 bit apps you may have installed on your computer. They will continue to work unless you upgrade your operating system to the next version. Contact the third party app developers for updates to those apps.

32-bit app compatibility with macOS High Sierra 10.13.4 and later - Apple Support

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

You're welcome.