Keep getting "the request timed out" when trying to download new update.

Are you using WiFi to download the update, if so you will have better success connecting to your router using a cabled connection.

When the mac is downloading don't use it or any other devices that share the same internet connection.

Firstly, check that your iMac is not pre-2012. Then follow this; https://support.apple.com/en-gb/HT201475