Click the three dots on the crop planting schedule in the sidebar
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In the drop down menu choose "Edit":
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The following "Edit Crop" form will pop up where you can edit the growing details for this crop planting schedule.

For more information on the details of what each section of the form mean see the article on Adding Crops to Your Calendar (there is a detailed description of what each input/toggle does at the bottom of the article).
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You can then edit or adjust any of the above planting schedule parameters and click "Save" to adjust the planting schedule in your calendar.