You can upgrade directly to 9.0 but few important points to remember..
1. Version 8.3 and up - 5520 needs 2GB memory. Make sure ASA got enough memory.
2. There are nat statement syntax changed from 8.3 on. IOS will convert the config but make sure you read and understand all the syntax changes.
3. Once you are ready, make sure you got backup of exsiting config (incase of roll back) and the procedure simply use a tftp server to copy IOS file to ASA flash and reboot.Check the below link.
You need access to the dowload section at cisco.com to do this. Contact your reseller or cisco to do this.
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