Dirty Hydrolic fluid

I just bought this machine and the hydro fluid is very dirty. I want to change it but what about all the fluid in the lines , cylinders, pumps , finals, etc... How do you get the entire system clean? Also is there any more return filters other than the large one in the tank? I have heard of a final drive return filter that must be changed/cleaned to stop pressure from building up in the drive motors.
2003 Volvo ec210blc
V11360

Thanks for any input

Dirty Hydrolic fluid