When changes are made to a post, the system automatically saves the prior version as a "revision," allowing you to revert to that version if necessary.
From the landing page, click the edit hover flyout and choose "Inspect revisions." (You can also get there by clicking the "Revisions" tab when in edit mode on a post.)
On the next screen, you'll see every revision made to that particular post listed.
From here you can inspect them one at a time by clicking on each datestamp, or you can choose two to compare.
Once you've chosen one to look at, click on the datestamp if you are not already looking directly at the one revision you have chosen. You can revert to the selected version of the post by clicking the Revert to this button.
You can also compare this revision to the current (published) version of the post.
Comparing two revisions lets you view the difference between them: