64 bit upgrade

Your computer is already 64 bit. Apple computers have been 64 bit for several years. You are likely asking because you got a popup about 64 bit. It is advising you that any 32 bit apps you are running will no longer be compatible with future Mac operating systems. So if you have any 32 bit apps installed, contact the app developers for upgrades of the apps.

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

You don't have to. 64-bit won't go into effect until the next OS after Mojave. All Apple app are already 64-bit and any 32-bit apps you have should notify you when there are updates.