When editing an existing event in a series of recurring events – events display all on one day
We’re working on a fix for this issue. In the meantime, you can create single events and then use a duplicate post/page plugin to duplicate the event. Then you will be able to edit your events without them getting scrambled up.
There are many duplicate post/page plugins available for free in the WordPress Plugin Repository, here is one for you to try: https://wordpress.org/plugins/duplicate-page/
NOTE: You should be able to make this change without any trouble. However, it is always a good idea to make a full backup of your site before making any changes.