Here’s how to do it if you haven’t upgraded yet.
Close Mail if it’s open.
Ctrl-click(or right-click) on the Mail Dock icon, highlightOptions, and then selectShow in Finder.
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Your Applications folder will open with Mail selected.
SelectGet Infofrom the Filemenu.
The keyboard shortcut for Get Info isCommand-I.
Expand theSharing & Permissionsmenu.
slide in your administrator password to authorize changes and clickOK.
see to it Read & Writeis selected foreveryone.
Find the desired icon in Finder.
Again, select Get Infofrom theActiondrop-down menu.
tap on the small icon in the info dialog of the icon file to highlight it.
Select Edit > Copyfrom the menu.
Now hit the small icon in the Mail Infodialog.
Select Edit > Pastefrom the menu.
It’s a security feature, but it will stop you from quickly changing the Mail icon.
Shut down your Mac.
Restart the computer while holdingCommand+Rto enter Recovery Mode.
Reboot your Mac and follow the instructions above to change the Mail icon.
see to it Read & Writeis selected foreveryonefollowing the steps above.
Highlight the small icon image in the dialog and select Edit > Cutfrom the menu.
Change the permissions back toRead-onlyforeveryone.
end the Mail Infodialog.
