Answers here are going to depend on your local laws, so let me try to give a general answer. However don't rely on anything I say being exactly legally true in your jurisdiction.
First thing to do is talk very strongly to HR. Tell them you expected the higher salary and that the lower is not acceptable. Ask for an explanation of the difference. Communicate with them in writing, accept only written replies, and keep copies of every communication.
Unless you want to just give up and accept the low salary, consult a lawyer immediately. They will be able to give you accurate legal information. Everything else I say here is stuff you should confirm with your lawyer.
Start looking for a new job. Unless you accept their terms there is very little chance that this ends well in the long term.
Normally an official offer letter, accepted by you, constitutes a contract. By changing the salary they are breaking the contract, and a lawyer may be able to enforce it, or sue them for the difference. Of course they can always fire you if they like, so enforcing the contract has little long term benefit for you. But until they fire you, a lawyer may be able to get you the original salary.
Another option is simply to walk away from the job. Write and tell them you are disappointed that the original job with the higher salary is not available, and you do not want the job at the lower salary. Do not say that you resign. Treat it as if they had withdrawn the job offer. Check this out with your lawyer.
Finally, tell anyone who will listen who this company is and what they did. If everyone knows about their underhanded tactics, people will stop being caught by them.