How to replace ssd from 120gb to 250?

It IS a possibility, as the actually flash storage module is removable, however please keep in mind that there are no 3rd parties that make replacement SSD's for the late 2013+ models of the macbook pro due to changes in the connection types.

My point is, you can replace it, but finding a part to replace it with will be very very expensive as the only parts available are ones from Apple, and one of the only places you may be able to find one would be on Ebay form someone who is selling just that part from another computer. If you find a replacement part or happen to have one then awesome go for it! However please keep in mind the third party parts out there, from Transcend and OWC, are only meant for the Macbook pro ret. models from early 2013- late 2012.

Hope that makes sense!