I had an offer which was paycut from my current salary. I had accepted that offer. Now a month later, I have got another offer, which has 20% pay hike from my current pay. What should I do please suggest?

Have you joined the offer with paycut? If not, just say you got a better offer and are withdrawing your acceptance. If you have already joined, there's generally a probation period of about 3 months where you can quit with a notice period of a week. This is two way of course as they can ask you to leave with just a week of notice. Anyway, you can probably just say you got an offer with work from home or something they are not willing to compromise over. You can also ask them if they can match your salary before you do that though. They just might.

offer letter is just an offer which you accept or agreed upon if you join Joining letter is something which you get once you join the company if you havent signed/accepted the joining letter, you can join any company and you are non bonded to the company to serve any notice period. if you have then you need to serve the notice period.