You can't upgrade the ASA5500 series to version 8.6.x.
Version 8.6.x is meant to be just the new ASA5500-X series.
Here is the release notes for your reference:
http://www.cisco.com/en/US/docs/security/asa/asa84/release/notes/asarn86.html
For the existing ASA5500 series (inc. 5510 and 5520), you can upgrade it to the latest version of 8.4.x.
Pls kindly be advised from version 8.3 onwards, you would need to have the upgraded memory on your ASA, and there are major feature change from version 8.3 onwards (ie: NAT and ACL in particular).
Here is the memory requirement for version 8.3 and above:
http://www.cisco.com/en/US/docs/security/asa/asa83/release/notes/asarn83.html#wp310503
Here is the complete release notse for version 8.3:
http://www.cisco.com/en/US/docs/security/asa/asa83/release/notes/asarn83.html
(Pls check out the new feature section).
And here is the release notes for version 8.4:
http://www.cisco.com/en/US/docs/security/asa/asa84/release/notes/asarn84.html
Hope that helps.