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