This installation description is for Ubuntu.

It is installed by default.

User name and email configuration (for all projects of that user).

git config --global user.name "Krzysztof Kowalski"

git config --global user.email k.kowalski@example.com

User name and email configuration just for that project.

git config --local user.name "Krzysztof Kowalski"

git config --local user.email k.kowalski@example.com

Back to the last commit, when unwanted changes are not commited and not added.

git checkout -- . and then git clean -df

Or

git reset --hard and then git clean -df

Or probably the best way:

git add . and then git reset --hard

Back to the last commit, when unwanted changes are not commited but added.

git reset --hard

Back to the any commit with working tree added to the index or not.

To be sure that all changes are added.

git add .

And then to go back to the pre-pre-last (second to last) commit.

git reset --hard HEAD~2