Increase WP memory limit with bluehost

I use WP 3.9.1 and my host is bluehost. Systems
diagnostic tells me that WP memory is limited to 40 MP. Bluehost uses
default settings which are 128 MB. I read documentation, saw bluehost
video but still don’t know how to increase memory. I also have other website on my bluehost accounts and don’t want to affect them by changing memory. Could you help me please