Lets say you are using the Flat layout then copy the file components/com_jevents/views/flat/search/tmpl/results_body.php to template/Y)OURTEMPLATE/html/com_jevents/flat/search/results_body.php and edit this file
Then switch the for look at line 29 to read
for( $r = $data['num_events']-1; $r >=0; $r-- ){
Note that it will only work when you are displaying ALL the results and not just some of them