Hello @Maoira thanks for the question, and sorry about the late reply! I did some research and, while you can most likely get a 6/7/8 speed cassette on your rear freehub (these speeds all share the same chain width and spacing), you would need to change the right rear shifter and rear derailleur, as both of these are only 6 speed compatible. Shimano does make 7 and 8 speed options. As @TomV said though, you may also need to get a new, longer chain and check for other compatibility issues. 

If you are interested in upgrading to a wider gear range, I would second @REI-AlyS' idea of contacting cyclinghelp@rei.com to speak directly with one of our technicians who can guide you to the right parts and even a local REI shop that could get you an idea of price and timeline. 

Hope this helps! 

Jim 

At REI, we believe time outside is fundamental to a life well lived.