Company switch in 2 days.

I recently joined a new company just 2 days ago, but I haven’t received my appointment letter yet. The HR mentioned that it would take around 15 days from my date of joining.

However, I received an offer from an MNC yesterday, and I would really like to join the new company instead.

How should I handle this situation in a professional way?

What are all the issue i needs to be faced?