You can only add new working times that do not overlap with any existing working times in the Calendar. 

If you receive an error that says "Start and end time must not overlap with any other existing working times in your profile", this means you are trying to add a new working time that is overlapping with an existing working time already entered in your Calendar. 

To resolve this issue, you need to either delete the existing working time in your Calendar that overlaps with the new time you are trying to add, or you can also edit the existing working time instead of trying to add a new one. 

If you are making significant changes to your availability, then it may be easier to delete all of the working times you currently have entered so you can start afresh.

Deleting working times will not affect sessions that have already been booked for your existing clients. You can toggle the different layers on the Calendar by using the Display Filters on the left-hand side of the Calendar page (more info here).