Just ctrl + z to undo whatever you regret having done, Nigel, or undo to get back to that point if you want to redo it differently. There's an "undo stack" built into the program, it's got a default 100-action level, if you've done that many in your editing session. Pro versions have some capacity to undo something without having to undo everything that came afterwards, that's what's called "history-based" CAD, and to carry a 100-level undo/redo stack from one editing session until the next time the file's opened.
Also, don't be timid about making changes to program or file. If you want to experiment, save your file under some name, bash away at whatever you think might work, and if it doesn't, close the file and reopen it from the saved version. Unsuccessful efforts are banished, you're back to a known starting point. Regarding the program, it's really, really hard, if not impossible to do irrevocable damage, so read the documentation, take a note of the steps that you take to do something, and if it doesn't work, the worst that can happen is that you suffer some inconvenience.