How to disable/remove 'RESET TO DEFAULT' button ?

i have this(below) in the admin file but its refer both to ‘save changes’ and ‘reset to defualts’ buttons.

#submitDiv input.button {
font-weight: bold;
font-size: 13px ;
padding: 7px 15px;
height: auto;
background: #2d2d2d;
-moz-text-shadow: none;
-webkit-text-shadow: none;
text-shadow: none;
color: #DDD;
border: 0;
-webkit-transition: .3s all ease-out;
-moz-transition: .3s all ease-out;
-o-transition: .3s all ease-out;
transition: .3s all ease-out;
}

You can remove it from the admin/main.php file (either by deleting HTML or adding CSS to hide it), but it will be restored on every update.

But why do you wish to remove it? You can create an export file to keep a backup of your settings if you’re afraid someone else with access to the site may use it to reset things.