how change 32 bit to 64 bit

BobTheFisherman has pretty much addressed your question. Is there a specific app or apps that are critical to you that are still 32-bit? If so, as Bob has mentioned, you will need to contact the app developer to see if they have provided a 64-bit update for their app.

Alternatively, you can run a VM manager, like Parallels or VMware, on your Mac. Then create a VM of Mojave or earlier to run these apps in.

You need to update your 32 bit apps to 64 bit. Contact the app developers for updates if they exist.

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