How to change network settings from g to n

Hello,

I am trying to figure out why my internet speed has changed from 300mbps to 54 mbps i am using a linksys dual band wireless N usb adapter and it was running at 300mbps and for some reason is now stuck on 54 mbps i have done all the speed test via the dsl web site and speed is 5.86mbps for download and 0.84mbps for upload i was told it could be that the network is on G and not on N how would i be able to configure it to N and also my wireless router is an N too i am currently running windows 7 hope this is enough info to help you solve my question
thanks sheri