How do I update an existing file?

[hxnk-57]

I want to create a new branch, make some code changes on existing files and then open a new pull request.

I have read this discussion #1152 and I am now able to create a new pull request.
However I am still unsure how to commit changes to a new branch and then opening a PR.

Are there any code examples that showcases some of these operations? I am not very familiar with Java so the documentation is not very intuitive for me to navigate.