Both phrases are correct.
The first one is for when you are wishing a person a happy birthday.
The second is used when you forgot someone’s birthday and want to wish them a happy birthday afterwards.
2020年11月12日
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The first one: belated happy birthday is correct.
2020年11月12日
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Hi Yuki,
'Belated happy brithday' is correct.
Jamie (www.italki.com/jamie.teacher)