Simply leave the work to the browser through a CSS Media Query:
@media only screen and (max-width: 760px) {
    /* esconde o elemento de controlo caso em Mobile */
    #el-controlo { display: none; }
}
On your page, you add an empty element that will be hidden when on mobile:
<span id="el-controlo"></span>
Finally, with jQuery you add the desired CSS class if your control element is hidden:
$( document ).ready(function() {
    // se elemento controlo escondido
    if ( $('#el-controlo').css('display')=='none' ) {
        // adiciona classe "dropup"
        $('#meu-elemento').addClass('dropup');
    }
});
The above example can be drastically reduced to a Javascript condition by resorting to Window.matchMedia():
if (!window.matchMedia("(min-width: 760px)").matches) {
    $('#meu-elemento').addClass('dropup');
}
							
							
						 
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– Guilherme Nascimento
Hello Philip! Please see here how to improve your question so that you get a good answer to your problem! If you can, paste the relevant part of your code and check the available formatting tools. Also, take advantage and do the tour, is worth a medal! D
– Daniel