use any file explorer with root access (root explorer) and navigate to the system folder.
you'll see on the top right of this program's interface there will be a little tab that you can press, you want it to go from r/o to r/w (read only to read/write).
in the system folder will be a file 'build.prop'. long press it and scroll down through options to open in text editor. you'll have to do some looking around but there should be a part that says vm heapsize=32mb or something like that. shouldn't be too hard to find. change it, save and exit and remount to r/o. changes are applied on reboot.