How to edit left menu links in SharePoint
Retrieve destination address
First you need to find the destination address of where you want to link to:
If linking to a page/library or folder: browse to the destination and copy the full address in the top navigation bar
If linking to an individual file: browse to the library it is in and click the 3 dots next to the file then select copy link
Editing/Creating new link
Click EDIT LINKS at the bottom of the left side navigation
Click the new link icon to add a new link to the list
Enter the name of the link which will be what it's called in the navigation side bar and paste in the address you copied previously.
Click Try link if you want to check it works then click OK to create the link.
The link will appear at the bottom of the list but you can drag it to the desired position (and re-order any others you want to change) then remember to click Save to finalise.