Below shows you how to upgrade your ASA to verson 8.04.

1. Enable scopy on your ASA 

firewall(config)#ssh scopy enable

2. Copy the image from your PC to the ASA, the command below is done on your PC via "Start / Run / CMD". Download pscp here.

pscp [image].bin [user]@[asa_ip]:[image].bin

3 .Change the boot order, so if the new image doesnt load, it will fail back and load the old image. 

firewall(config)# boot system disk0:[new_image].bin
firewall(config)# boot system disk0:[old_image].bin
firewall#sh run boot

4. Reload the ASA, once rebooted check the version.

firewall(config)# reload
firewall#sh ver