I look after a garden for a couple with a labrador that keeps marking the grass. It's unsightly, but it is easily fixable.
Step one - with a hand rake clear out and remove the old, yellow grass.
Step two - sprinkle on a suitable product - I use Miracle Gro Patch Magic which contains grass seed, feed and coir.
Step three - water well, and keep the area moist, making sure it doesn't dry out.
Step four - wait for your new grass to grow - within a couple of weeks you will see new shoots.