Currently it is not possible to move a candidate back to a previous online assessment stage but the interview request isn't affected. The candidate will still have the same opportunity to complete the interview from the previous stage.
If you accidentally moved a candidate to the following stage all that happens is that they may get a notification that they've progressed stages (depending on your notification settings for the stage you progressed them to). Info on emails sent to candidates can be found here.
We are working on adding an "undo" option after progressing for situations like this.
In the meantime, if you are concerned you can always contact the candidate by email to ask how their interview is going. Please note that they have up to 4 days to complete the interview and you can check the time left in their candidate card.
For complete info on how candidate cards work, please look at this article here.