Let me explain with pics...pics help make everything easier.
This is a computer/browser instruction and may/may not be different for those of you using Tapatalk.
At the top-right of your COAR page in your browser window, click on Settings
Scroll/look down the menu list on the left side of the page under My Settings, then Miscellaneous you'll find Attachments - click on Attachments
From here on it should be intuitive.