I have an 32GiB USB flash drive.
I wrote Debian Live image to USB flash drive by "dd".
Now on USB flash drive 27 GiB free space.
I want to use this free space.
Try using a partitioning tool like gparted, available on the excellent SystemRescueCd, to access your USB flash drive. Assuming your Debian installation is on the only partition on the flash drive (/dev/sdb1 or something similar - use lsblk and/or fdisk -l to check), you can use gparted to shrink the partition and create a second partition (or a number of them) in the 27GB of free space.