It's not possible.
Some cable internet providers and / or cable modems have UL/DL limits, even if your lease is faster.
For instance if you pay for a 10 Mbps line, your download speed in ideal conditions would be about 1200 kBps, but a cable modem might have a 512 kBps cap. It's possible to uncap a cable modem, 'let the beast out', but it's a procedure that can go really wrong.
You can not uncap DSL / ADSL / VDSL modems or routers.