If you need to delete or make an amendment to a date withing a recurring absence, you will need to break that date away from the rest of the sequence.
To do this, click on any date in the sequence. This will open the booking form at the first date in the sequence. You will then need to change the end date of the recurrence to the last date the absence would have occured before the date you wish to change.
Once you have done this, create a new recurring custom absence that starts on the next occurence after the date you wish to amend or delete. You will then have two separate recurrence sequences that fall either side of that date.
From here, you can either leave that date clear if you just wanted to delete the absence on that day, or you can enter a single day absence on that date that will be independant of the recurrences.