You have to try yourself for drafting such letters.
Anyway, you may consider the following format.
QUOTE
To,
Dear Mr. /Ms,
In recognition of your outstanding performance in your current job, we are very happy to inform you that you have been promoted as “designation “with effect from “due date”. In accordance with this, we hereby revise your Gross Salary as Rs /-.Other terms and Conditions mentioned in your Appointment letter dated ... shall remain unaltered.
Kindly sign and return the duplicate of this letter as a token of your acceptance of the above terms and conditions.
Congratulations!
With best wishes,
Very truly yours,
Authorized Signatory
UNQUOTE