This is not a mod_security related question.
Login with ssh as root to your server.
Type the following command to edit the php.ini:
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# nano /opt/alt/php-fpm70/usr/php/php.ini
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(or any other php.ini file you are using)
Find the following two values (with [CTRL]+W):
post_max_size = 100M
upload_max_filesize = 100M
Set them to any value you want.
Save and close the file. Where,
Restart Apache server:
$ systemctl restart apache2.service
Now you should able to upload bigger files.