Keras Online Learning probem in implementation

Question

I want LSTM to learn with newer data. It needs to update itself depending on the trend in the new data and I wish to save this say in a file. Then I wish to call this pre-fed training file into any other X,Y,Z fresh files where testing is done. So I wish to 're-fit' [update NOT re-train] the model with new data such that model parameters are just updated and not re-initialized. I understand this is online learning but how to implement it through Keras? Can someone please advise how to successfully implement it?