1. First, decompile the apk file and decompress it into an editable file. You can use some tools, such as APK change theory, ApkTool and so on, to achieve this step.

2. Secondly, modify the content in the apk file and replace or edit the corresponding file as needed. You can modify resources, codes, configurations, etc. In apk file. If you want to modify the code, you need to compare the smali file with the Java source code and modify it according to smali specification.

3. Finally, repackage the apk file and recompress the modified file into apk format. You can also use some tools, such as APK change theory, ApkTool and so on, to achieve this step.