for example.,If you have apps icons like "icon.png" and "icon.svg"

if u don't have .svg format icon just convert or get from anywhere because icon.svg format is must 
step 1: 
copy both icons and go to addon module -> static-> description-> (icon.png, icon.svg)paste here ......
which module u want to display that icon
step 2:
already default icon will be there so you need to rename it and reasrt ur server and upgrade that module then the icon will display in your apps
step 3:
after put that icons its not display then go and check the module menu items web icons format
like if the icon format is icon.png u need to change icon.svg
then save and upgrade ur app module.. then u will get the result finally
step 5: don't forget to check icons file path in web_icon in the module root menu item