If you have older hardware, you need to first verify you have enough memory to support the new image. Here's a link to the guide for that.
Then you will have to perform a 2-step upgrade - first to 8.3 or 8.4 and then to your 9.0 target. First refer to the document "Cisco ASA 5500 Migration to Version 8.3 and Later" and follow the procedures there.
To go the last step, follow the procedure in the 9.0(x) Release Notes.
One thing I would add in addition to what the guides cover - make sure you have a backup including the VPN pre-shared keys. If you copy off your config using the following, it will print them out in plain text:
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more system:running-config
Hope this helps - please rate helpful posts.
