Enable:

enable

Make sure there is enough space on disk0, delete for example the ADSM image.

dir
delete disk0:asdm-711-52.bin

Download new image.

copy http://1.2.3.4/asa/asa904-k8.bin disk0:asa904-k8.bin

Go into configure mode.

conf t

Show boot image.

show version | i disk0

or

show running-config boot system

Remove current boot image from config:

no boot system disk0:/asa901-k8.bin

Configure new image as boot image:

boot system disk0:/asa904-k8.bin

Save config:

copy run start

Reload:

reload

Almost the same procedure as above:

enable
copy http://1.2.3.4/asa/asdm-731.bin disk0:/asdm-731.bin
conf t
asdm image disk0:/asdm-731.bin
copy run start
reload

Verify version numbers:

sh vers | i Version
Cisco Adaptive Security Appliance Software Version 9.2(2)4 
Device Manager Version 7.3(1)