I haven't missed a primary since 1966 when I turned 21. They are the only source of change.
Primaries. Vote in the primary. Only a small percentage actually vote in them. Often times it's those connected to the political party machine. So it only takes a few thousand like minded folks to boot out the old guard. Just ask Eric Cantor who thought he was the next speaker of the house a few years back.
A few hundred folks voted for another guy and Eric was done before the November election that year. His replacement did more good for the country than Eric did. Trouble was the other side figured it out and nominated a wack-o disguised as a moderate and Dave Brat was defeated in the next cycle.