How do I replace a brake Master cyclinder?

Having to ask suggests a lack of familiarity with cars but it is something you can do yourself with some precautions and warnings. First of all, it is absolutely imperative you do not allow any hint of grease or petroleum product to get on anything that contacts brake fluid. I even wash my hands to avoid contaminating the brake fluid with fingerprint grease. Use a flare-nut wrench, also called a line wrench, to loosen and tighten the soft nuts holding the steel lines to the master cylinder.

I'll include a trick that will prevent you from having to bleed at the wheels. Loosen the line nuts. Remove the nuts holding the master cylinder to the power booster. Pull the master cylinder off the booster and use it as a handle to bend the steel lines upward just enough to keep the fluid from running out of them. Remove the nuts the rest of the way. Don't let dripping brake fluid get on paint; it will eat it away.

Put the new master cylinder in a vise to hold it, then "bench-bleed" it. The new part will come with instructions for doing that, and a pair of hoses. Run the line nuts into the ports by hand. Those nuts should spin at least two or three turns to be sure they aren't cross-threaded. Push down on it to bend the lines back the way they were, then bolt the master cylinder to the booster. At this point you'll need a helper to work the brake pedal. The line nuts must not be tight yet. Have your helper VERY SLOWLY push the brake pedal half way to the floor, (never all the way). It should take about 20 seconds. As he does, you'll see air bubbles coming out around those nuts. When he hollers that he's half way down, or when you stop seeing bubbles and only see fluid, snug the line nuts, THEN he can release the pedal rapidly. If he releases the pedal before the nuts are tight, air will be drawn back in. By pressing the pedal slowly, any air in the lines will keep floating back up as the fluid goes down to the wheels. By releasing the pedal quickly, any trapped air bubbles will just wash back up into the reservoir along with the returning fluid. Loosen the nuts and do that a second time to be sure no more air bubbles come out. Tighten the nuts and fill the reservoir according to the condition of the front brake pads. If the pads are fairly new, fill the fluid to the maximum line. If the pads are older, fill the fluid to a little more than the minimum line. There needs to be room left for the fluid that will return there in the future when new pads are installed. Wash any spilled brake fluid from painted surfaces.