If i decide to change one of focus result to critical hit result, must i change other result to hit results? I use Marksmanship on Kath Scarlett and sometimes want to change only one focus result to critical hit result for a bigger chance to cancel this result and to use Scarlett's ability.

The "You may" part is right at the beginning of the ability, stating that you may use it (a choice). However, it does not say you may change 1 of your eye's to a crit and then you may turn all your other eyes to hits. If you choose to use the ability, you resolve its effects exactly as specified on the card.

I concur, It's all or nothing.

You may use the card, or not. But if you do so, you must apply all its effects.