Change Feature definition XML programmatically

Hello.

I need to change feature definition xml programmatically. To be more specific, I have a feature on my sharepoint 2010 server. It contains few list and content type definitions. I want to remove a column from one of the list in feature and create other. But I didnt find how to get access to existing feature xml content to do that

I found how to get all fetures of the site collection

foreach (SPFeature feature in site.QueryFeatures(SPFeatureScope.Web, true))
                {
                    SPFeatureDefinition def = feature.Definition;
}