I have still (?) from G parted checking installed Ubuntu 18.04 on sda1 and KALI on sda2. Dual boot is not working anymore system goes straight to Ubuntu. How can I install safelly UBUNTU STUDIO on sda2 insteda KALI so that system will boot? Also how to make rescue USB in case non booting?
1) Make sure you have copied off all data from Kali's partition you might find useful.
2) Delete all the contents of sda2 after booting from the Ubuntu in sda1 .
Create a LiveUSB with Ubuntu Studio after confirming the download of Studio matches its checksum.
3) Shutdown and then reboot with the Studio Live USB.
4) Choose install and select Something Else when it asks for where to install it.
5) Should GRUB2 not give you the option to boot Studio after install completes and the PC reboots, perform a Boot Repair with https://help.ubuntu.com/community/Boot-Repair