I have WebSphere Application Server v8.5.5.0 installed on my machine and want to upgrade WAS to v9.0 without uninstalling the current version. How can i do that using the command line as after we successfully upgrade on local machine we need to upgrade WAS on Production Linux server as well. Can anybody help on this with steps?
You can install two versions of WAS side by side, that'll work fine. From there, you can migrate your profiles over using WASPreUpgrade and WASPostUpgrade; I use the steps here: Migrating a cell using the command line tools. Make sure to take good backups of your environment, in case you want to roll back or re-do part of the process.
The command reference for WASPreUpgrade and WASPostUpgrade are useful as well.
Personally, I'd also make sure you install the latest fixpack for 9.0 that you possibly can--each one resolves a bunch of possible bugs and pitfalls.