How to define a variable already defined in a plugin function?

This is fairly easy. First, you need to use a child theme. See the following on child themes if you are unsure what they are or how to use them.

https://developer.wordpress.org/themes/advanced-topics/child-themes/

The theme you are using uses pluggable functions which makes them easy to override.

Note the function in the theme is written like this…

<?php
if ( ! function_exists ( 'my_function' ) ) { // This is the function name 'my_function' you will use in your child theme
    function my_function() {
        // Contents of function.
    }
}
?>

All you have to do is create your own function in your child theme with the same name — like this.

<?php
function my_function() { // Note: This is the same function name 'my_function' used above
    // Contents for your function override here.
}
?>

Thus all you need to do is add this function to your child theme.

<?php
function mkdf_hotel_room_get_currency() {
    $currency = '€'; // default currency if woocommerce is not installed
    if ( fivestar_mikado_is_woocommerce_installed() ) {
        $currency = get_woocommerce_currency_symbol( get_woocommerce_currency() );
    }

    return $currency;
}
?>

As per your theme updating concern it will not be affected, due to the fact that the change is in your child theme not the parent theme.

Good luck hope this helps.

Best.

Tim