They have apps that do it but in my opinion easiest way is to do it by hand cause you can then set whatever you want so i just use root explorer then go to the build.prop file this is located at /system/build.prop
long press the build.prop and select open in text editor
scroll down a bit till you see
# This is a high density device with more memory, so larger vm heaps for it.
dalkvick.vm.heapsize=<some number>m
Then just change that some number to what you want the recommended usually is 32 i think but i just go big
and put it at 52 ha just so i could run miui player and what not but if you feel more comfortable using an app just go to the market and search vm heap size there are a few apps that can do it for you.
Can also be done through terminal but that's a bit more crazy works a little like this though just for knowledge purposes here's a quick script to change it to 52
#!/system/bin/sh
#
BB=busybox
$BB sed -i "s/dalvik.vm.heapsize=.*/dalvik.vm.heapsize=52/g" /system/build.prop
$BB sed -i 's/hw_dalvik.vm.heapsize=.*/hw_dalvik.vm.heapsize=52/g' /system/build.prop
then you would just save that as a file and place it at /system/xbin make sure to set your permissions and then run it through terminal emulator by just typing su and then the name of whatever you named the script