Can I cancel a trade?

[thisismeusernameything]

Hello, quick question that I hope somebody can answer.

I have a paper share certificate. Yesterday, I agreed to sell the shares via a broker over the telephone.

Today I have found a much cheaper way to sell the shares and would like to cancel the trade if possible.

The contract note arrived today for me to return the share certificate but it doesn't say anything about rights of cancellation.

Can I cancel the trade?