To keep the changes intact, you will need to use a child theme. If you don't have one yet and don't know how to create it. please follow this guide first: How To Make A Child Theme

Once you created and installed your child theme, add the following code to the /functions.php  file in your child theme folder:

/* DON'T copy the first line (above) if your functions.php already has it.
* ------------------------------------------------------------------- */

function et_projects_custom_slug( $slug ) {
    $slug = array( 'slug' => 'custom-slug' );

    return $slug;
add_filter( 'et_project_posttype_rewrite_args', 'et_projects_custom_slug', 10, 2 );

Replace the custom-slug string in the code above with anything that you want (ex: book ).

Lastly, you might need to refresh your permalinks for this change to take effect. To do this, open your WordPress Dashboard and go to Settings > Permalinks then simply click on the Save button:

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