How to change kernel security level?
If you have just run sysctl kern.securelevel=1 to change the securelevel, as you have indicated, then you just need to reboot your computer. This way of changing the securelevel is not persistent, so you will be back at securelevel 0 after rebooting, so that you can remove the system immutable flag from the file.

If you have changed for example /etc/sysctl.conf or used some method of ensuring that securelevel is set to 1 at every boot, you could edit that file and then reboot to get back to securelevel 0.

If you just want to temporarily get to securelevel=0 in order to remove the flag from the file, I would suggest booting into Recovery mode by holding down the Command and R keys while booting the Mac. While in Recovery mode, open Terminal and run chflags to remove the flag from the file.