Acrylic paints are fast drying and when dry are no longer water-soluble. This is why it is very important not to let the paint dry on your brushes. It is also more difficult to remove acrylics from natural bristles over synthetic bristles.
Rinse the brush regularly while you are using it but do not leave it soaking in water. When you have completed your painting session, wash the brush in cool (not hot) soapy water to create a lather, ensuring there is no paint at the ferrule end and between the bristles. Rinse well, squeeze out excess water and reshape the brush. Leave to dry horizontally.