How Many Days Before Joining Can I Negotiate Salary After Accepting an Offer?

I recently accepted an offer from a company and have received the offer letter. However, I am expecting another offer from a different organization which might be more aligned with my expectations. I wanted to know:

How many days before the joining date is it appropriate to negotiate the salary with the company whose offer I have already accepted?

Are there any best practices or tips to handle this situation professionally without affecting the relationship?