Kai, I’m afraid only the first one works. Or better yet, "can I exchange it if it doesn't fit?" (The clothing fits, not the size.) Alternatively, you could ask "can I exchange it if the size isn't right?" To talk about whether or not clothing is the right size for one’s body, we use “fit” as a verb, not as an adjective. As an adjective, “fit” has a much more general meaning, for example, a person can be “fit” (in good physical condition, or suitable for a job, etc.), or a meal can be fit for a king (i.e. “suitable”, “appropriate”). Exchange: to trade one (garment) for another Change: in addition to the meaning of "exchange", can also mean to alter the same garment

The first is right. 'Can I change it if the size does not fit me?' ... you can also say ... 'Can I change it if the size does not fit?' The second is wrong. You would need to add more to it. 'Can I change it if the size is not the right fit?'