You want to replace the social icons with the adsense horizontal ad right?
The easiest way to do this is:
Go to header.php, take backup of this file first.
Replace line 53 to line 70 with your adsense code.
Now instead of social buttons it will show adsense ad.
Now in custom.css paste
.socialmedia {width:70%;}
.themonic-logo{width:28%;}change the width parameter to your liking depending on your logo and ad size.