First update appId in your capacitor.config.ts file. Then, if you are using VS Code, use the Ionic Extension - Configuration -> Properties -> Bundle Id.
If doing it manually, for Android, see Change application name on android platform - #2 by twestrick
For iOS, I believe it is just the PRODUCT_BUNDLE_IDENTIFIER (two places) in ios/App/App.xcodeproj/project.pbxproj.