Not sure anyone from WP Job Manager will help you since this is a “customization” question and they don’t seem to take kindly to folk asking questions or not knowing their code inside and out. Since I just had to figure this out I can tell you what I did.
You need to do a template override.
The button text is located in the file:
wp-content/plugins/wp-job-manager/templates/job-application.php
Make a copy of that file and place it in a new directory under your themes folder called “job_manager” You’ll need to make that directory.
example:
/wp-content/themes/yourtheme/job_manager/
Once you copy (not move) the file in that new directory, edit the new file and look for this string:
<input type="button" class="application_button button" value="<?php _e( 'Apply for job', 'wp-job-manager' ); ?>" />
Change the text “Apply for job” to whatever you want and save the changes. Do not remove the start and ending apostrophes.
That’s it!