My question is, currently i have a harddisk of 1 tb in which i have given 40gb space to ubuntu and 410gb for /home and another ext 4 partition of 510gb for my own user data. Now i want is to delete ubuntu and install kali without losing my own user data of 510gb.
Your question seems to imply that your disk is partitioned, and that the Ubuntu OS and /home reside on their own partitions. Assuming that this is so:
Take a backup.
Remove the Ubuntu and/or /home partition.
Follow the installation instructions. At step 9, select Guided - use the largest continuous free space.
Other options for creating free space are shrinking the /home and user data partitions, or consolidating and shrinking them.