You could change it however I haven't done so myself and I would not recommend doing it unless you are a professional electrician since you would be messing with the only way of getting an internet connection.
Based on your stats, your noise margin is quite high typically it should be around 7-12dB. That being said you stated that you live about 3 miles from your exchange therefore this is acceptable. One issue that occurs to me is that your router is reporting a maximum speed of 3071Kbps download this equates to about 3Mbps. This would reflect the speeds that you are receiving.
You will most likely not achieve a constant 3Mbps since it would make the line unstable therefore the exchange will lower the speed slightly in order to maintain stability
Could you unplug and plug your router back in and see if this changes? If not then you may want to ring your ISP about the speeds you are receiving. Especially if you are paying for a higher speed when you physically won't be able to get. Please note that the speed that your router is reporting may be affected by capped speeds from ISP and the only way to see if you have been capped is by contacting them.
My apologies for not being able to offer a solid solution on your end since internet speeds are out of mine or yours management and can only be configured by your ISP