Choose the 1st digit any of 9 ways (It cannot be 0)

Choose the 2nd digit any of the 9 remaining ways (It cannot be the digit 
chosen for the 1st digit, but it can be 0).

Choose the 3rd digit any of the 8 remaining ways (It cannot be either of 
the 2 digits already chosen).

Choose the 4th digit any of the 7 remaining ways (It cannot be any of 
the 3 digits already chosen).

Choose the 5th digit any of the 6 remaining ways (It cannot be any of 
the 4 digits already chosen).

Choose the 6th digit any of the 5 remaining ways (It cannot be any of 
the 5 digits already chosen).

Answer 9*9*8*7*6*5 = 136080 ways.

Edwin