You use one of the following programs:
- fdisk (command-line utility)
- gparted (GTK partition editor, for Gnome/XFCE/..., graphical)
- qtparted (QT partition editor, for KDE, graphical)
- some other program that can handle partitions
If the automatic install used LVM and your system is now on logical volume groups, you can't use the above tools (only, at least). Instead/in addition you need a program to control LVM, but that's probably in your distribution already, if it uses LVM.
Look into the administrative menus.