Retrofitting is actually addition of new technologies to old structures. The old structures may be vulnerable or may be not. It can be used to increase the efficiency or make it economical, It may also decrease emissions.
However from Civil engineering point of view The Retrofitting term may be referred to Seismic Retrofitting, it is the modification of existing structures to make them more resistant to seismic activity, ground motion, or soil failure due to earthquakes.
I would recommend you to read the following popular techniques for seismic retrofitting:-