Welcome to Flyertalk
2 possibilities :
- upgrade with FB miles. You need to call FB to do it to see if there is availability
- paid upgrade at OLCI (cash or mix cash+miles), if there is space in W and if you are targeted by an offer. You may also ask at check-in. You can expect something in the 300-400 EUR range.
Or you can change now your ticket to a W one, by paying the fare difference between Y and W.