For Shared and Reseller customers you can easily do this in cPanel by taking the following steps:
Log into your cPanel account
Under the “Software / Services” section, click on “Select PHP version”
On the right hand side, click the link that says “Switch To PHP Options”. If this says “Switch To PHP Extensions” then you are already in the right spot
Click the gray setting next to the following option:“memory_limit” – A drop down will appear and allow you to change the memory allowance per PHP process.
For VPS / Dedicated customers:
Log into WHM as root.
Click on “PHP Configuration” under the “Service Configuration” menu
Fill in the circle next to “Advanced Mode”
Scroll down and look for “memory_limit”. You can change this variable to anything you want however you should never exceed the amount of your server. In most cases, 128M will be fine.
Click “Save” at the bottom and make sure to restart Apache by clicking “HTTP Server (Apache)” in the left menu under “Restart Services”.