Well, this is actually not too hard and ext4 gives better performance in various cases, so an update to ext4 is surely not a bad thing to do, but first check if your kernel does support ext4!

Converting an ext3 to ext4 is plain forward: 

1. unmount the file system
2. tune2fs -O extents,uninit_bg,dir_index /dev/sdaX
3. fsck ( not 100% necessary, but doesn´t hurt )

So here you go:

cd /; umount /dev/sda1
tune2fs -O extents,uninit_bg,dir_index /dev/sda1
fsck -pf /dev/sda1

If you want to boot from an ext4 root device, you have to tell this to your kernel via boot parameter:
rootfstype=ext4

If you use grub as boot loader, change the entries as this one:

title		Debian GNU/Linux, kernel 2.6.26-2-686
root		(hd0,0)
kernel		/vmlinuz-2.6.26-2-686 root=/dev/mapper/gosisrv01-root ro quiet rootfstype=ext4
initrd		/initrd.img-2.6.26-2-686