"UPDATE tblHOURS SET [Responsible_Supervisor] = '" & Me.cmbRS & "' WHERE [ActivityID] = '" & Me.cmbAID & "'"

Most developers would save an ID (EmployeeID maybe) for the responsible supervisor instead of their name. Name parts really should be in separate fields: FirstName, Middle, LastName. This will insure consistency. It is easier to concatenate fields then to split up text.