Your mixin should look something like that:
@mixin respond-to($media) {
@if $media == smartphone {
@media screen and (min-device-width: $break-smartphone) { @content; }
}
@if $media == smartphone-only {
@media screen and (min-width: $break-smartphone) and (max-width: $break-tablet) { @content; }
}
@if $media == tablet {
@media screen and (min-width: $break-tablet) { @content; }
}
@if $media == tablet-only {
@media screen and (min-width: $break-tablet) and (max-width: $break-widescreen) { @content; }
}
@if $media == mobile-only {
@media screen and (max-width: $break-widescreen) { @content; }
}
@if $media == widescreen {
@media screen and (min-width: $break-widescreen - $webkit-scrollbar-width) and (min-device-width: $force-tablet ) { @content;}
}
}
$media
is a var/placeholder for other Sass vars, in case of this mixin $break-tablet
is the var you want to change.
So just put
$break-tablet: 360px;
somewhere in the SCSS file you're including the mixin to (doesn't need to be above inclusion) and you'll be fine.
Another question is, why you'd like to include a breakpoint of 360px? There's no common tablet using 360px width. It's foremost (older) smartphones. But you'd be excluding iPhones with 360px.