How can override a add_filter of a plugin?

Filters come with a priority parameter, the default is 10, so to override a function you need to increase the priority:

add_filter( 'wcml_switch_currency_exception', 'cart_switching_currency', 99, 4 );

add_filter( string $tag, callable $function_to_add, int $priority =
10, int $accepted_args = 1 )

More info in the add_filter

Update:

If removing a filter is not working you may try this approach:

function remove_cart_switching_currency_filter(){
    remove_filter('wcml_switch_currency_exception', 'cart_switching_currency', 10, 4);
}
add_action( 'after_setup_theme', 'remove_cart_switching_currency_filter' );

Important is that the priorities must match.

More info about after_setup_theme