I want to update the string length for a shapefile layer in QGIS. I've tried opening up Layer Properties and clicking edit but it won't let me change it manually. Also I've tried using the field calculator but I can't seem to get my head around it. I can create a new comment field (see "Comments_1"), but can't update the existing "Comments" to a character length of 254. The screenshot shows both windows I tried in.

Any help would be much appreciated!