Refer to documentation from vendor
https://documentation.brightspace.com/EN/le/manage_dates/instructor/offset_dates.htm
Bulk Edit Dates
On the navbar, click Course Admin.
Click Manage Dates.
Select the check box of the items for which you want to bulk edit dates.
Click Bulk Edit Dates.
In the Bulk Edit Dates dialog box, update the due dates and availability dates.
Click Save.
Offset Dates In Manage Dates
On the navbar, click Course Admin.
Click Manage Dates.
Select the check box by any item for which you want to offset dates.
Click Bulk Offset Dates.
In the Bulk Offset Dates dialog box, enter your offset details.
Click Save.