Is this in the Joomla search or the JEvents internal search

By newest do you mean the order or creation or the order of event start date?

In the JEvents internal search, i want the search order with event start date from the newest to oldest. But i can't find the file to change the search parameters.

I'm sorry 'newest' = nearest to now, 'oldest' = furthest into the future or 'oldest' = furthest into the past ?

sorry, like from the future (the first one of the list) to the past (the last one of the list) (anti-cronological) using start date as parameter.
Like the actual search, but invert the order.

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