Well, the problem could either be with the modem itself, or it could also be due to the network issues. Either way, I do not think there is any way you can improve the speed on your own. Here are a few suggestions -
Contact customer care
Register a complaint with AirTel. Please note that AirTel is quite notorious for lack of response for speed related complaints. Use multiple channels to place your complaints. Complaining on their social media channels can have better feedback.
Check your network zone
Not all cell sites ( called towers in layman's language) in India have been upgraded to 3G or LTE. Check if your area has better coverage. If not, you are bound to have slow speed. It would be advisable to opt for another service provider with better coverage in your area. Check if you can unlock your AirTel USB modem. If it can, you use any other SIM with the modem. Please note that the steps involved may damage the modem. Expert advice is needed for the procedure.
If everything fails, consider surrendering your AirTel connection. Please note that Air Tel has many harrowing stories of continued billing even after closing the connection. Follow the requisite procedures carefully to close the connection. Once you submit an application for the termination of the services, DO NOT use the modem - even for checking. AirTel has been known to coming up with claims that the service is under use.
Live....and Let Live!