Click any article title in the calendar to open Joomla's standard article editor, the same screen you would reach from Content > Articles. Make your changes and save as usual.

Closing the edit screen takes you back to the exact calendar view you started from.

Normal Joomla permissions apply: you can only edit articles you have edit rights for. If a title isn't clickable, or the edit screen shows a permission error, check the user's access level for that article's category.