If you have one of our cPanel plans, in order to update the memory limit for drush, create or edit the following file in your home directory:
~/.drush/drush.ini
Add or change the line at the bottom of the file
memory_limit = -1
Save the file, your new limit is applied fro Drush/CLI immediately.
If you run cPanel with CageFS and have root access to your server, you might be looking for the How to install Drush in cPanel with CloudLinux and GageFS article