Move the mouse over an observation in the Phase Window. Then do a right mouse click to bring up the context menu. Select Set epoch to set the epoch time (= starting time for calculating the phases of a Phase Window) to the Time value of the selected observation. The new epoch time will be reported in a dialog box, and the Phase Window will be redrawn using the new epoch time.

The Set epoch command is only enabled when you prompt the context menu while the mouse cursor is over an observation.