You can change the PHP version of your hosting account and set several variables like memory_limit, max_execution_time and post_max_size.
To be able to change this variables, login your cPanel > Software > Select PHP version
If you are using Native version of PHP then select the version you want to use from the dropdown menu, you can select the same version already used but not the native version, Native version is the PHP installed server wide and you can't set PHP values if you use this version.
in screen shot below, the current version of PHP is native (5.4), and user is selecting PHP 5.4 but not the native version, after choosing the right version, click on "set as current" button
Once you set the right PHP version, click on "Switch to PHP options" link
Options page shows the available variables and their values
You can click on any value to edit it
That's it, changing the value will take immediate effect.
Please note that we put limits on each website resources (depend on the package) to protect the server resources, so you need to choose PHP values wisely, usually default values are enough, consider optimizing your website resources usage before chaning PHP values specially memory limit and execution time.