I want to change keyboard from JP to US

You can replace the keyboard keycaps Vs the entire keyboard. While you will have some limitations here it is the cheapest way. Here's a good YouTube vid on how: Removing & replacing a single key. Here's one source replacementlaptopkeys.com - MacBook Air

Replacing the keyboard really requires replacing the entire top case assembly (some have has success getting the keyboard part out of China directly). Removing the keyboard requires removing a lot of snap rivets and/or screws (aprox 50) Here's a YouTube vid with someone taking a similar keyboard apart (one with screws) MacBook Air '11 keyboard removal if you do try that route Vs replacing the whole top case. Replacing the entire top case is the only way I recommend to do this if you do need to replace the keyboard.