How to edit/add to promoted links (the tile icons used for site navigation)
Navigate to the site / subsite where the promoted links appear
Click the settings cog and select Site contents
In Site contents click on the name of the list
The tile view of the list should then be displayed.
Click the List tab at the top of the page and change the Current View drop down to All Promoted Links
Click new item to add a new entry to the list or edit to make other amendments to the list
Background Image Location is used to display a custom graphic for the tile. It is recommended these are stored in the Site Assets folder in the Site Contents folder of the site they are to be used on or the home Site Assets folder if the graphic could be used in different areas.
Link Location is where you want the link to take the user to. This can be to an internal area of SharePoint or external location
Note: In order to complete the Background Image Location and Link Location fields it is easier to open a new tab so you can find the address and paste it into the list when required