Hello, Vladimir!
Here are the codes to change the signs:
nav.mobile-menu>ul.menu>li.menu-item-has-children>.dropdown-toggle.collapsed:after {
content: "\f107"!important;
}
nav.mobile-menu>ul.menu>li.menu-item-has-children>.dropdown-toggle:after {
content: "\f106"!important;
}
you can choose the content code from the following list:https://fontawesome.com/v4.7.0/cheatsheet/