DBIx::Class comes with lots of documentation. I guess you will have to read it, instead of asking us to read it for you.

In your specific case, I would assume that you would first ->search() for the rows you want to change, and then do $resultset->update(), to change these rows.

But, to reiterate, you will have to learn lots and lots about DBIx::Class. Or, if you "just" want to write some SQL, you will have to ditch using DBIx::Class as your model in Catalyst and write your own model. This should be possible, but I haven't done any of this, as I find that writing my own framework is about as much hassle as using one of the premade frameworks.