Firmware upgrade

Login into ASA (SSH or serial port)

Provide 'enable' password

enable

Prepare files on TFTP server.

ping 192.168.74.128
copy tftp: disk0:
dir

Perform change of booting image

show version
configure terminal
dir

show bootvar
no boot system disk0:/asa845-k8.bin
boot system disk0:/asa917-32-k8.bin

exit
write memory
show run | include boot
show bootvar

reload

show version