Microsoft has recently announced that it has been rolling out an update to select Lumia devices in India to enable 4G LTE support. The list of devices include the Lumia 1520, Lumia 1320, Lumia 1020, Lumia 930, Lumia 925, Lumia 920, Lumia 830, and Lumia 625.

Microsoft rolls out its software update to select Lumia devices to enable LTE support in India

You will need to go to Settings > Phone update > Check for update to install the update (or wait for the notification to appear on the phone). Once installed, go to Settings > Mobile + SIM to enable 4G if your network provides this service or just leave it on 2G/3G to save battery.