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

If you need to update the instructions or the contact for a particular gig, 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 gigs that have not been filled, and book new gig(s) for a different shift directly from your Instawork dashboard.