If you want to change it for orders that are yet to be placed, then changing the Purchase option’s settings through your Preorder app (assuming you have one) is the way to go. On our app for example - you can change the fullfillment date for a product through the preorder offer’s settings and that applies it to all orders coming up. This works the same across most preorder apps.
If it’s for existing orders, I think you’d need to do this through Shopify APIs/an app. I generally look to Flow for cases like these, but it doesn’t seem like they have a general action for Fulfillment Orders. (Maybe someone with better knowledge here can point out a Flow based solution 
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