Airtel subscribers need to change their old SIM to 4G ready SIM in order to use 4G on their supported smartphones. That's why Airtel has started a process using which Airtel customers (both prepaid and postpaid) can swap their old SIMs to new 4G ready SIM very easily. This activation process takes very short time around 10 - 20 minutes.
If your city is covered by Airtel 4G, then you can visit this link and fill out the details mentioned there and get a new 4G ready SIM at your doorstep for free. And after swapping your old Airtel SIM to the new 4G ready SIM, you can surf the the internet on 4G LTE.
Airtel also provides 4G ready USIM with various 4G compatible smartphones bought from e-commerce site like Snapdeal, Amazon, Flipkart etc. This way even in non 4G areas Airtel customers can use 4G ready SIM in advance and when Airtel launches 4G services in those areas its customer will be ready to take the full advantage of high speed 4G services. These 4G ready SIMs come in two adapters making it compatible with both Micro & Mini SIM adapter based handsets and are also backward compatible that means normal 2G or 3G networks will work without any disturbance.
Airtel 4G SIM Swap Procedure
Step 1. Send an SMS "SIM<space>20 digit SIM no" to 121
**20 digit SIM no will be printed on the back of the SIM or on the SIM packet
Step 2. You'll get an SMS from Airtel acknowledging the swap request. Now reply with 1 to confirm
Step 3. Now wait for few minutes while your old Airtel SIM gets disconnected from the network, once that happens insert the new 4G ready SIM into your phone and that's it.