Updating to a 64 bit in Word

Update your software.

In regards to the popup window—this is a generic warning only— You have to do nothing, 32-bit apps continue to work on High Sierra and 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 have an Office 365 subscription or are running Word 2016 (or later) for Mac on your Mac, then it is already a 64-bit application.

You need to upgrade to the latest Word.

https://products.office.com/en-us/word

You might try asking your Microsoft product question in the:

Microsoft Product Help Communities