How to manipulate timeouts in Handler

I have set read and write timeouts to 5 seconds. But I have handler responsible to add data from excel which takes more than 5 seconds to complete the request. So what I want to do is change or increase the read/write timeout by X amount of time if the request is not completed within 5 seconds. I have implemented TimeoutHandler provided by net/http package to cancel the request if it is not completed within specified time. What are the ways to do this? How can I manipulate the timeout?