Upgrading drive train for Cannondale Trail 6
You will need to change the rear shifter, also best to install new cables (inners and outers) to get the most out of the upgrade. 

No need to stick with your existing brand. Microshift do a range of well priced products that would suit that bike very well. 

In Shimano, XT quality is probably spending too much for that bike, Deore is practically the same quality  the same just a bit heavier and a few less features (e.g. brake levers on Deore miss out on the tool-less reach adjust and miss the bite ajust)