Every space ships with a versions button next to the save menu. By pressing it you will see your current, and all previous published versions. Once you click on an older version of your content entry you can see the “Restore” button on top of a git style display of changes. Yellow means that an attribute was changed where the previous content is struck through, the currently active value will be shown after that. Green indicates an empty field was filled with content or a new field was added to the schema. Red stands for a delete, where one field or block was removed.
Once restored you can than edit the old version. If you want to go back to the current version you can do the exact same steps and choose the version before the current one as it should be the version before you’ve restored the old one. That is true as long as you did not publish the older version, if so you need the check the versions before that as well.