How do i export data to excel file in memory and download it

Question

I have Excel Template file. What i need to do is I need to export data from database
to the file and download the file on button click.

Right now what i am doing is i have created a processor that copy the
template file and export data and makes it ready to download for the location.

But instead of running processor and having file copied and saved in different
location, I want to copy the template in memory and export the data in memory and response the stream out.