How to "undo" a commit?

[Christian Hujer]

Hello.

I have just committed some bunch accidently. (Thanks god this is the first 
time this happened after some years of cvs usage)

I have no idea about tagging or branching. I know the latest revision is 
always called HEAD.

What is the best way to make the HEAD revision of the files being the previous 
revision? I've read the faq, they say if the last version is 1.31, "checkout 
1.30 and recheckin as 1.32". But I cannot imagine how to do this.
When I try "cvs update -r 1.30 file" and then "cvs commit file" it of course 
does not work.
Or is there a possibility to mark the 1.30 version as HEAD?

Christian