Slower upload speeds than advertised

When I look at my current internet plan it says I should have up to 400 mbps download and 100 mbps upload, but I only have 10 mbps upload at the moment.  I live in Minneapolis so I believe the area should have the upgraded upload speed and I have XFI Complete.  Also I only have the XB3 modem, so I'm wondering if I just need to go into an Xfinity store to switch it out for the XB6 modem to get the faster upload speeds.