You simply have to make a new will stating that this will supersedes you earlier will (refer the date of the earlier will) and any other testament or disposition and that the present will is your last will and testament.
It is very important that the will be attested be two witnesses other wise the will be held invalid.
Also it is important that the will be drafted in language known to the testator (person making the will). If it is not drafted in the language known to the testator, there should be advocates seal confirming that the contents of the will have been explained to the testator in language known to the testator.
When you execute a fresh Will, the earlier Will would get cancelled automatically. There is no separate document for cancelling a Will. Usually a Will commences with these words..."THIS IS THE LAST WILL AND TESTAMENTARY WRITING OF ..........................." Similarly the will usually contains a paragraph ...."1.	I hereby revoke all other WILLS or CODICILS made and executed heretofore and declare this WILL to be my LAST WILL AND TESTAMENTARY WRITING."
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