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You don't want to write a letter, you want to file a petition for modification of support order. In the petition, you simply set out the facts: That the original order provided that your ex-spouse was allowed to claim the child, that you now provide 90 percent of her support (assuming that is correct), and that based on those facts, you are asking the court to provide that you be allowed to claim your daughter as a dependant on your taxes.
First, you should address it to the judge in the court that originally granted the divorce. Second, there is no special format you use if you are simply doing a letter, although I would use numbered paragraphs, and simply gothrough the points that I listed in my original answeer:
1. Original order provided husband was to claim child as dependant.
2. Circumstances have changed and now you provide more than half of the child's support.
3. You request the court to modify the original order.