You may change the number of units in which you are enrolled for courses approved for variable units in CalCentral. Increasing and decreasing a unit value equates to adding and dropping courses/units, therefore deadline and fee policies for both Add and Drop actions apply when you change a course's unit value.
Log in to CalCentral and navigate to the My Academics tab and select the Enrollment Center option on the Class Enrollment Card.
To adjust the course units (if allowed) select the Preferences/Switch Sections option. Select the course you would like to adjust and follow the prompts.