A field that was edited for a single task in a series will not be overridden if you edit that field for the entire series later on, for example: If you changed the title of an instance to "A" and then you change the title of the series to "B", the title of above instance will still be "A".
Step by step:
Sign in to your Guesty account.
From the top menu, click Tasks.
Select the relevant task.
To the top-right, click Edit.
From the drop-down, select Edit series. Please note that if you edit the "Timing" field, all upcoming tasks that have not yet started will be deleted and new tasks will be created based on the new Timing.