How can I use a video header that’s over 8MB?

I’m using a theme that supports WordPress’s (relatively new) video header feature. I want to upload a video that’s over 8MB big, and I don’t want to reduce the size of the video. How can I force it to accept a larger video? Is there a file that I can edit or a plugin that I can download?

The limit is enforced by _validate_header_video method, registered as validation callback on respective Customizer setting.

There is a filter that allows you to hook into the process and manipulate those arguments:

add_filter( 'customize_dynamic_setting_args', function ( $args, $id ) {

    if ( 'header_video' === $id ) {
        // change $args['validate_callback']
    }

    return $args;
}, 10, 2 );