Changing Internet Plans in first 30 days

I Made a mistake and got 600mbps for a 300 mbps modem and I need to switch but do I how to do it Im not sure do I have to Cancel my services and re order for 300 Mbps to do so or is there another method? also I activated my account and services too early in accordance that the The Modem hasnt been severed from previous Internet connection yet will waiting when it does disconnect automatically connect my services to the modem or how do I go about this please help?