[Flow] how to upgrade from flow 2.1 to 2.2 / 2.3

[Mira]

Hi there,

Actually I'm running flow 2.1:

root at websrv:/var/www/flow3/green# ./flow help
TYPO3 Flow 2.1 ("Development" context)

Now I want to upgrade to the latest version of flow 2.3.1.

So I have downloaded the base distribution from http://flow.typo3.org/download
and read the included Upgrading.txt.

But I don't know how to upgrade my existing and working installation of flow.

In the Upgrading.txt is only described how I migrate my existing code. But what
I should do before running the migrate commands?

Should I replace all the files of old ones with the downloaded base distribution
(of the newer version) in my project directory? Before I have to migrate?

If I tried this and run the migrate commands after this, I got many error
messages like "the working copy is dirty".
What be the right way to upgrading my flow project?

Thank you for your help.

Mira