Simply replace the unfilled areas such as "[]" and "[date]" with your information.
Dear [name],
Contained herein is the [identify product], purchased on [date], which is still under warranty. I have enclosed a copy of my bill of sale for this purchase.
The problem that I am experiencing is as follows:
[specify]
I would be most appreciative if you would either make the necessary repairs or send me a replacement.
Thank you for your prompt attention.