i've Debian installed RAID 1 2x 160GB HD, i want to delete it but i'm totally noob in linux, i don't want to install fresh image and start from scratch again, i cannot do backup because i've nearly 140GB already used.
There are a few ways your system can be setup to boot, but a common method using mdadm is to boot read-only from a single device, then remount the root read-write as the raid pair.
It is most likely that editing your /etc/fstab to repoint /dev/mdadm* to /dev/hda* will break the pair. After that, /dev/hdb* will be a backup of where you were when you rebooted. You can obliterate and use as you wish.
Keep in mind that there are many ways in Linux to create a software raid pair, so measure thrice then cut. LVM is different than mdadm, etc.