This short tutorial shows how to resize a ext4 root filesystem online. Most people use gparted from a recovery system for this task, and I also recommend that for if you are not familiar with the tools used in this guide. Also, make sure you have a backup of the data (I did this myself on a system I just installed, so I would not mind if anything goes wrong). Generally this is not different from doing a manual MBR/fdisk resize, except that we need to treat the unique partition GUID specially since we should maintain it (for boot loaders/boot managers).
So, here is how I resized my ext4 root partition (sda2) using gdisk and resize2fs: