The Package Manager will lock the Git package to a specific revision in your project.
This is to render the resolution of your project dependencies deterministic.
It is a mechanism to protect users against out of scope changes that could break your project.
To update the project to a different revision, you have two choices.

1) Re-add the Git package URL in your project through the Package Manager UI
2) Open the project manifest (Packages/manifest.json) in a text editor, erase the entry for your package in the lock section.

Hope this helps