You can prune lilacs now, it is not too late. The ideal time to prune is right after they've flowered, so if it's been a few weeks, you may not get as many flowers next spring. There are several ways to prune lilacs depending on how big they are: there is renewal pruning and restoration pruning. Here is an Extension publication that goes into detail on how to prune your lilac:

Growing lilacs for Minnesota landscapes | UMN Extension