Click on the General Graph Properties icon   in the Basic Toolbar.

Select whether to show the Well Sticks and Annotation and set the parameters for the wells.

Select the Horizon(s) to display.

Select the Seismic Data to restrict the horizons to.

Once a horizon graph is displayed, use the Horizons for Picking drop down  to select the horizon to edit.

Select either Straight Line Picking  or Manual Picking from the Horizon Toolbar.  The displayed horizon will turn yellow to let you know it is in editing mode.

Once the editing is finished click the picking mode off to turn the horizon off editing mode.