Look at the label on the bottom of your router. It should explicitly state Ver: 3.0 or V3 .
For Indian users, the direct link to the support page is: https://www.tp-link.com/in/support/download/tl-wr850n/#Firmware Tp-link Tl-wr850n V3 Firmware Download India
Navigate to > System > Firmware Upgrade (or System Tools > Firmware Upgrade depending on your interface). Look at the label on the bottom of your router
However, you may still find archived firmware via: Tp-link Tl-wr850n V3 Firmware Download India
: Connect your PC directly to the router via an Ethernet cable . Do NOT attempt to update over Wi-Fi, as a connection drop can brick the router.