how to update my the sims 3

I believe The Sims 3 is a very old app and is likely only a 32bit app. 32 bit apps are not supported on macOS 10.15 Catalina. You will most likely need to purchase a newer version of the Sims game, but make sure it is compatible with your model Mac and version of macOS. You always want to meet or exceed the Recommended System Requirements.

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

See the link under the Related Article above and contact Electronic Arts, the vendor for the SIMS product family.