Removing Install Operating system from download area.

I have a 15" mid 2011MAC whose hard drive crashed. I could do nothing with the laptop after that. I had a newer 13" MAC laptop, which I used to try to create a bootable disk coffer the 15" MAC along with the Last Operating system that was on it. I found the instructions on the web site and followed what I though was exactly what was necessary. I subsequently decided not to update the 15", but recycle it. I am trying to remove the install High Sierra OS system which I downloaded to the 1e3in, but am unable to do dos. I transferred the Install named above from the download area of my 13" laptop, but a window appears stating that it can not be removed Because that item I asm trying to install "Mac/OS High Sierra is in use". How can I delete this item????