You should do that through a Vendor pricelist.

In the Purchase app go to vendor pricelist and filter for your vendor and edit the Vendors Delivery lead time.

Thanks

Vishal

No you can't do it (or complex to do).

You have orders and those all are scheduled based on lead time and some of processed, some partially processed and some done. So you can't do it.

Also you have to recompute all the dates depending on Lead time.(only change the value in lead time will not recalculate all the dates which all are already scheduled).

Instead, change the lead time in respective products and next orders are scheduled based on this date.

(for new yes, by setting lead time on pricelist)