In the maintenance plan simulation graphic, choose  Edit Move.

Select the colored block representing the date you want to shift and move it to the required date using the left mouse button.

Release the mouse button.

A dialog box appears asking you whether you want to shift the date selected and all subsequent dates, or whether you only want to shift the selected date.

Choose the shift type you require. You return to the maintenance plan simulation graphic, on which you can see all the dates which have been moved. The color of the block has also changed, so you can see that this is a simulation.