To change “back” to another text in mobile menu navigation, you can use below CSS code.
#1. All pages
Use this CSS code
a.header-menu-controls-control.header-menu-controls-control--active>span:nth-child(2) {
visibility: hidden;
}
a.header-menu-controls-control.header-menu-controls-control--active>span:nth-child(2):before {
visibility: visible;
content: "new back";
width: 100%;
}

#2. Specific Page
First, you need to find the Page ID.
In my example, it is:
- #collection-6673f2e18432c25013aee99f

Next, use this CSS code
#collection-6673f2e18432c25013aee99f {
a.header-menu-controls-control.header-menu-controls-control--active>span:nth-child(2) {
visibility: hidden;
}
a.header-menu-controls-control.header-menu-controls-control--active>span:nth-child(2):before {
visibility: visible;
content: "new back";
width: 100%;
}}

Result:
