Hi DUSHYANT
You can do it from Stock Central that allows you to edit multiple products at once and to save them all at once.
But this is a task that must be done manually (as each product may have distinct stock).
Alternatively, you can use our ATUM Export Pro add-on and the Import (that we will release soon and will be part of the Export Pro), so you can export a CSV, then to edit the stock for all the items there and importing back the file to your site.
I hope this helps.
Best Regards,
Salva.