There is no way, Although ....
Say, If you are getting Signals(WCDMA 300(20~50kbps) kbps or HSDPA 3.2(200~350kbps) MBPS) from Tower A then there is no way you can change to HSPA(6.0(450~850kbps) MBPS), because that Tower A might not have HSPA technology.(6.0(450~850kbps) MBPS). So you have to move around and find yourself a suitable tower which can provide you good signals as well as have good HSPA or HSPA+ type technology.
Signals switches very fast as it depends on a ratio, accord to which your device(receiver) is forced to choose nearby towers..
So you cannot fix your modem or device to a tower you like(there is :p but very complex search for AT LOCK FREQ).
Best way is to travel around the city and find a suitable tower yourself.....
If you can change your thread title to 'How to get good speed from modem?' or alike wise, it might be helpful to our forum as these discussions are worldwide, and people asks these types of questions.