If you need to adjust the hours for a shift you already created, just follow the instructions here.
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If you need to add Instawork Professionals to your shift, you can do so within the dashboard on the Instawork website by clicking the "Add Professionals" button on the the page of the shift you need to modify. 

If you need to update the instructions or the contact for a particular shift, you can do so within the dashboard on the Instawork website like in the image below:
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Additionally, you can cancel open shifts that have not been filled and book new shift(s) for a different shift directly from your Instawork dashboard.