You must have the specific rights to modify a CRM activity. If you do not have these rights, you can request them from your management.
In the following example, we have filled in this visit as of August 9 by mistake. It is now possible to change it to the correct day by clicking on the event. In this case, the following box:
Then click on this button to reopen the event
Change the event, in this case we simply changed the day.