IMO the most natural way to express this is to start to allow the syntax
UPDATE foo SET X = X + N WHERE key = K;
(for counter column X)
The alternatives would be to represent it as a procedure call:
SELECT increment(foo, K, X, N);
I don't like that much though, since it's a semantic mismatch (there is no actual function named increment involved). I'd rather save that for when we actually introduce functions/stored procedures.