Changing content from an unpushed git commit in Visual Studio

You can use amend within VS.

Change what you want to change and in the Team Explorer - Changes window click Actions and Amend Previous Commit.

This will change the last commit and rewrites it with your changes (amend).

On the CLI, this would look like the following: git commit --amend (with options to change the message too)
The documentation is pretty good on that topic:

• Git Basics – Undoing Things
• Git Tools – Rewriting History