Hi @nav18004 . The best option that I have found, at least on a PC, is to push "CTRL+Z" which is the undo shortcut. I believe the equivalent on a Mac is "Command +Z". You might give this a try and see if it works for you.
All the best!
To add to ericwerth's advice (who just beat me to this! ) you may also want to vote up on this Feature Idea and read some of the information contained in the comments about how to activate another menu bar in the rich content editor: https://community.canvaslms.com/ideas/4782-create-an-undo-button-in-content-editor