You can edit submission fields with 2 different methods:
Via Custom Code
Edit job submission fields: Tutorial
Edit resume submission fields: Tutorial
The tutorials on how to edit the submission fields are linked to the WP Job Manager documentation.
Via Third-Party Plugin
The best and most recommend way is via the third-party plugin WPJM Field Editor. WP Job Manager Field Editor is the only plugin that will allow you to completely customize all submission forms (jobs, resumes & cariera companies) without the need of writing a single line of code.
Video Tutorials: Playlist