No way to choose, it will normally update the one in the application folder (I think).

But if you want to be a bit more sure, copy either the whole folder plus all the related, files (some are hidden) to an external disk and disconnect that. Or keep the updater to 7.21 if you still have that so that you simply can go back via fresh installation.

To see what is all related, you maybe look into Chipps thread here at Renderosity (from last week I think), where he has a link to his blog with an overview where is what in an installation.

Oh, and please, if you update, check the preferences. Here they were messed up a bit so that things became a bit weird.

One day your ship comes in - but you're at the airport.