Installing operating system in place of existing one

Dell Inspiron 15 3542 Intel i3-4005U comes with Ubuntu OS.  I want install Debian in place of Ubuntu.

Will be better completely format hard drive when install Debian, or its make sense keep existing HDD paritioning structure and install Debian into Ubuntu partition (Partition3)? There is 5 partitions:

DellUtility, Partition 1  (367MB), FAT
OS, Partition 2 (3.2GB), FAT
Filesystem, Partition  3, (488GB), Ext4
Extended, Partition 4  (8.4GB)
Swap, Partition 5, (8GB)
Free space 1.1MB