Inside Anattella, the undo functionality is available in all text fields: Simply press CTRL-Z to undo (and CTRL-Y to redo).
If you made a mistake while editing the data-transformation-graph in itself, simply close the Window that contains your .anatella file (use the 
button) and re-open your .anatella file: See section 8.5. on how to quickly close and re-open Anatella graphs.
By default, Anatella saves your .anatella files each time you run them. It means that, if you did a mistake while editing your graph, you should close it (and therafter re-open it) BEFORE running it.
Also, you can ask Anatella to keep backup copies of your graphs. Using a previous backup of your .anatella file allows you to “go back” several steps “in the past”. You can define the quantity of backups. A large number of backups allows you to keep track of all your modifications and editing over a long period of time: See section 7.1. about .anatella file backup.
Also, if you want to “go back” even more in the past, you can use a versioning system (such as Git) to keep track of all the versions of your .anatella files over the course of several years: See section 8.9. about versioning and collaborative work.