Update version:A.B.C+X in pubspec.yaml.
A.B.C represents the versionName such as 1.0.0.
X (the number after the +) represents the versionCode such as 1, 2, 3, etc.
When you run flutter packages get after updating this version in the pubspec file, the versionName and versionCode in local.properties are updated which are later picked up in the build.gradle (app) when you build your flutter project using flutter build or flutter run which is ultimately responsibly for setting the versionName and versionCode for the apk.
A.B.C represents the CFBundleShortVersionString such as 1.0.0.
X (the number after the +) represents the CFBundleVersion such as 1, 2, 3, etc.