hello @Liz93
please Go to Online Store->Theme->Edit code then go to assets/theme.css ->paste below code at the bottom of the file.
@media only screen and (min-width: 750px){
.site-header {width: 100% !important;}
.site-header__logo-image {max-width: 235px !important;}
.site-header__logo-image >img {
max-width: 240px !important;
}
}