First of all, do not do this from a running system, do this from a live-CD. Download for example the GParted live-CD and mount it to the virtual machines CD drive, then boot the VM from that CD. Now you can easily use GParted to shrink the /home partition and extend the /-partition.
If you really want to merge those partitions you first have to copy the contents of the /home partition to a separate drive, then delete the /home partition, extend the / partition and copy the data back to the /home folder. Keep in mind that you also have to adapt your /etc/fstab in this case.
Tip for the future: You can avoid situations like these with using LVM instead of ordinary partitions.