Just writing the body the of the letter
My dear uncle,
You are more than just uncle for me. You are an inspiration and perfect example. You are one of the most amazing people that i have ever known.
I hope your birthday's filled with love, joy, laughter, happy memories, wonderful moment and perfect in every way.
As you are my special uncle for me, it is so nice to have this chance to wish you the best of everything.
Even though you are great to everyone, it seems like you treat me the absolute best! That's the reason I love you a lot.
I feel very lucky to have a uncle like you. As my vacations will start I will come to meet you.
Your lovingly niece,