You can replace the words by editing the source file.
1. Remember to backup the original file before any update.
2. Login to the admin dashboard.
3. Navigate to Appearance->Editor and select the file section-upcoming_search.php on the right.
4. You will see a code like below:
wp_dropdown_categories(array(‘name’ => ‘event-category[]’, ‘taxonomy’ => ‘ja-event-category’, ‘selected’ => 0, ‘class’ => ‘category chosen’, ‘show_option_all’ => __(‘By Category…’, ‘januas’)));
5. Just replace the words ‘By Category…’ with the words that you want.
6. Update the file.