Which part of the letter needs revision in order to be appropriate for a formal letter?

Depending on the letter, you would need to make sure that the language is revised to be less causal. If you have contractions, change those to written words. For example: "Can't: changes to "cannot."

If the greeting is something casual like "hey" change that to "Dear Sir" or "To whom it may concern."

Also please note your closing greeting is "sincerely" or something of a similar stroke.