How do I clean dried brushes?
You need a Brush Cleaner and Restorer formulated for this task. There are a few of these available, only in art supply stores; you'll want to check with the ones around you to see what you can purchase.

If you are in the United States, Winsor &amp; Newton makes a widely available product I have used with great results; it's formulated for dried acrylic and oil paints, but I have also used it to dissolve away dried latex (house) paint, as well as years-long set dry varnishes. With this product, you'll let the brush soak up the solution, checking periodically to monitor the progress. In a low quality brush, it may end up damaging it by damaging any glues holding the bristles in place in the ferrule, but at the point you're using a restorer on a brush, it's typically ruined anyway, so there's no risk in trying. In my experience, I've only had one or two brushes lost this way out of dozens.

Unfortunately, this particular product is not available outside of the United States, but other companies do make similar products that you'll be able to find by asking your friendly local art supply shop. The employees there should be able to recommend a product to you with which they're familiar.

Ultimately, the best option is to avoid needing a restorer in the first place; take proper care of your brushes, cleaning them right away instead of letting paints dry in them. Avoid getting paint up into the ferrule, and dry them bristles down so paint and water don't soak down into the ferrule and cause damage.