I'm not sure what you're asking. If you mean that:
- You published an app to the Market, at version 1
- You upgraded the app, in the market, to version 2
- You discovered that that upgrade had a bug in it and you now want to publish a new upgrade
If that's the case then you have to post a new version as version 3. However, remember that there are two things you control when posting new versions. There's the versionName which can be anything you want and then the versionCode which must be a number and each release must have a higher number than the previous one. The versionCode is how the Android Market knows if the version in the Market is more up to date than the version on someone's phone. The version name is just the name people see when looking at the app in the market. So in your case you must change the versionCode to 3, but you could keep the version name as "version 2".