You can set Amazon Restock Date individually per product or in bulk via a CSV file upload
Please go to inventory-> inventory & listings -> edit the relevant product
Next select the stock levels tab-> enter the re stock date-> finally select update quantity.
For Bulk Update, Please go to Bulk Actions -> Import and upload a General CSV file using sku,restock_date value. Please note that the date format should be yyyy-MM-dd. (e.g 2023-06-22)