I have read software development lifecycles and different approaches for the initial project that needs to be developed. Can anyone suggest and explain different approaches for a software upgrade or update to an existing project?
Agile methodologies can be used here as well, if you want to implement incremental updates. It is especially important to make smaller changes to an existing code base, since you can break old functionality, if they are not guaranteed by tests.
If this is something you are doing frequently, "Working Effectively with Legacy Code" might have some more answers.