This question already has an answer here:
I work on QGIS 2.2, and I am unable to edit my attribute table to change the field type from string to double.
I have gone through several tutorials. - I can not run the table manager plugin, probably because its latest version is for qgis 2.0, (well it appears broken on my plugin manager UI).
I am unable to edit my layer so that I can use the field calculator (as several tutorials stipulated).
I am not versed with python to use the python console