If you're not sure how to ask the question, then you probably haven't learned the fundamentals of git. I would recommend reading the free Git Book, especially the first three chapters.
To answer your question directly:
If you have not committed the bad changes, you can use the Discard button, and choose the Reset All tab to clear out all local changes.
If you have committed changes, and HAVE NOT PUSHED THEM, then what you want is a "hard reset". You do not remove files from your machine. Rather, you instruct your repository to replace your working copy with the files from an existing commit. In SourceTree, you can right-click the "good" commit in the graph, select "Reset to this commit", and choose the "hard" option.
If you have pushed bad changes to the remote repository, then the situation is more complex. Try searching "how to revert pushed changes in git".
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