How to redirect a 404 to another url same domain?

[aldragon]

Hi,

Have the following problem

need to redirect an old url from my website to a new one

old was:
http://www.metamercade o.net/2010/09/informe-seo

new is
http://www.metamercade o.net/posicionamiento-google

How and where do I do this
thanks

[Markus]

This would be better done through the web-server. What software are you using?