In the illustration the matches represent hurdles and the coins/counters are sheep. 
The sixteen hurdles on the outside, and the sheep, must be regarded as immovable; the puzzle has to do entirely with the nine hurdles on the inside.
These nine hurdles enclose four groups of 8, 3, 3, and 2 sheep. 
The farmer requires to readjust some of the hurdles so as to enclose 6, 6, and 4 sheep. 

Can you do it by only replacing two hurdles? 
When you have succeeded, then try to do it by replacing three hurdles; then four, five, six, and seven in succession. 
Of course, the hurdles must be legitimately laid on the dotted lines, and no such tricks are allowed as leaving unconnected ends of hurdles, or two hurdles placed side by side, or merely making hurdles change places.