To increase max_execution_time you need to change your PHP configuration file php.ini
Please find in this file line
max_execution_time=120
change it to
max_execution_time=300
Another way to do this it modifies .httaccess file (this is not always worked)
.htaccess file
<IfModule mod_php5.c>
php_value max_execution_time 300
</IfModule>