It is possible to edit previously created events on mobile.
From the top menu of the page, choose the day on which the event has been saved on. You can view more days from the calendar icon at the top right corner of the page. 
Click on the title of the event to open the edit view for that event. In this example case, the title Work is selected.
In the edit view, you can edit the event the same way as in the desktop version of the calendar. You can edit event type, start and end times, notes, projects (if enabled), overtime targeting (if the person has rights to specify overtime targeting), and supplements (if enabled).
When the wanted changes are made, click Save. The changes are now saved.