How to Un-Cap a Cable or DSL Modem
"Cap" is a reference to an artificial limit on downstream and/or upstream speeds. Such caps are common on consumer-grade service. "Un-capping" is thus an attempt to remove such limits (and thereby increase speed).
However, un-capping is an urban legend (or hoax, take your pick) -- subscribers cannot un-cap cable or DSL modems:
Cable Modem speeds are controlled by the CMTS (head-end box for Cable Internet)
DSL speeds are controlled by the DSLAM (head-end box for DSL)
(Note that if you did somehow find a way to do it, that might be construed as theft of service, with unpleasant consequences.)