In addition to Nick's technical answer, I want to add the philosophical angle -- Why would you want to modify a field during template execution? While technically this is possible, it makes no sense to do so while the template is executing.
I understand you want to change a field value, then save the Component to propagate your change to the CM database. This is a valid use-case... just not while templating. The template itself is a text (sometimes binary) transformation of some content. The analogy here would be someone making an XML - XSLT transformation and attempting to change the source XML while transforming. Technically possible? Sure. But it makes no sense, since that's why you are performing the transformation in the first place.
You can use event system if you want to validate some values or change them upon saving.
You can use GUI extensions or custom URLs to pull in different values or change/format values.
You can use the Core Service to change field values programmatically from some content import application.
Perhaps you can detail your use case and I'll try to come up with the right solution.