You can cut off the foliage in late fall almost down to the ground if you like - it will grow all new stems next spring anyiway. I'd give it a small handful of slow-release fertilizer to munch on over winter. Coreopsis are tough and durable. They do eventually die of old age, though, so I like to scatter some seed from the dried-up flowers so be sure to have new plants next year - but then my garden's not super tidy. The plants transplant well, too.