If you haven't added an Actual Return to work date (and the status is still Initial report or updated report) then you can remove it by finding it on the list of sicknesses on the Dashboard and clicking the Sickness Reference to open it.  You will find a Remove button at the bottom of the Sickness.

If the Return to work date has been added then you will need to Kill it.  To do this you will need to know which employee was sick and the numerical part of the sickness reference (e.g. SR001234 = 1234).  Then go to Admin > System Functionality > Sickness > Kill Record.