The post_max_size directive can be set in .htaccess, php.ini , .user.ini or wp-config.php file. To gain access to these files you will need FTP credentials. If you do not have these please contact your hosting provider.

1. In your WordPress root installation locate any of these .htaccess, php.ini, .user.ini or wp-config.php
2. Edit the file and paste the directive code
3. Once you are done making changes save the file. If you have downloaded the file to make this change, upload it back to your server and override the existing file.

Directive for .htaccess

php_value post_max_size 32M

Directive for php.ini or .user.ini

post_max_size=32M

Directive for wp-config.php

@ini_set( 'post_max_size' , '32M' );