Follow the below guide to perform batch update on product quantity
Step 1: Navigate to Catalog > Products
Step 2: Select the products by click the checkbox that you want to update the quantity, click on Actions drop down then select Update attributes
Step 3: Select Advanced Inventory menu, on the Qty field, tick the Change checkbox then set the Quantity amount. Once done, click Save button to apply the changes.
NOTE: The more product you selected for batch update, it will take longer time to complete.
Once done, need to re-index and flush Magento cache for the updated record to reflect on live site.