To upgrade ASA-OS first download new image to disk0: (flash) for example from ftp server. After downloading, list the disk directory and make sure right image is on:

ASA# show disk
--#-- --length-- -----date/time------ path
95 69454656 Sep 17 2021 09:53:56 asa941-lfbff-k8.SPA
96 26350916 Sep 17 2021 09:54:42 asdm-741.bin
97 33 Jul 20 2018 15:22:49 .boot_string
11 4096 Sep 17 2021 09:57:28 log
20 4096 Sep 17 2021 09:58:34 crypto_archive
21 4096 Sep 17 2021 09:58:52 coredumpinfo
22 59 Sep 03 2021 09:58:52 coredumpinfo/coredump.cfg
98 41836544 Sep 03 2021 09:43:30 asasfr-5500x-boot-5.4.1-211.img
100 16962269 Apr 29 2018 15:11:35 anyconnect-win-4.1.06013-k9.pkg

Set the required image to boot variable:

ASA(config)# boot system disk0:asa941-lfbff-k8.SPA
INFO: Converting disk0:asa941-lfbff-k8.SPA to disk0:/asa941-lfbff-k8.SPA
ASA(config)# sh boot
BOOT variable = disk0:/asa941-lfbff-k8.SPA
Current BOOT variable = disk0:/asa941-lfbff-k8.SPA
CONFIG_FILE variable =
Current CONFIG_FILE variable =

You can reload the ASA and wait that the new ASA-OS boots up:

ASA#reload