Place this function within your themes functions.php file or within a site specific plugin.

functions.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

function mycustom_filter_gettext( $translated, $original, $domain ) {
 
    // This is an array of original strings
    // and what they should be replaced with
    $strings = array(
        'Please choose a payment option' => 'Please select the Paypal icon below to be taken to the Paypay site for payment',
        // Add some more strings here
    );
 
    // See if the current string is in the $strings array
    // If so, replace its translation
    if ( isset( $strings[$original] ) ) {
        // This accomplishes the same thing as __()
        // but without running it through the filter again
        $translations = get_translations_for_domain( $domain );
        $translated = $translations->translate( $strings[$original] );
    }
 
    return $translated;
}
 
add_filter( 'gettext', 'mycustom_filter_gettext', 10, 3 );