Tools you need :
1) Cheat Engine 5.5 or Here
2) Firefox
3) Flash Player 9 or 10

1) Go into Happy Pets
2) Select firefox in Cheat Engine
3) Change value type to "Double"
4) Scan current amount of coins.
5) Spend some or earn some and NEXT SCAN new amount of coins.
6) 1 address returned else repeat step 5.
7) Change the value of this address to any amount you want.