I use a syringe to pull out the old fluid from the reservoir before adding fresh new brake fluid. You might find that there is a little gooey dark stuff down in the corners of reservoir. I add a little new fluid then swish with a Q tip and syringe that stuff out to start fresh with everything clean. Fill the reservoir with new fluid and bleed at the caliper until fresh new fluid is coming out.
This might help: 12,000 mile Brake Fluid Replacement
Edit to add: I posted that DIY when I owned a non-ABS 2013 model. ABS bikes 2014 and newer can use the exact same sequence, ABS makes no difference or difficulty. 2012 and 2013 ABS bikes have a different front caliper that has a different bleed sequence, adding one step for the foot brake to bleed the center front piston.