How to shut off this water softener bypass valve

Typically, the valve solders to the 1/2 inch water source pipe. the outlet is a pretty standard 3/8" compression fitting. It is a little hard to tell from your pics, but it is rare that the valve would be a threaded fitting to the water source, but not unheard of. The differences between the valves pictured are that the top pic is a 90 degree 1/4 turn valve and the bottom pic is a straight multi-turn valve. If you are going to replace them, do yourself a favor and use nothing less than a 1/4 turn stainless steel ball valve. They are available in both straight and 90 degree styles.

Assuming they are solder on, you must heat them to melt the solder and remove them, or cut them free with a tubing cutter as close to the existing valve as possible. If you unsolder them, take great care to remove all the old solder and clean/smooth the pipe end well before attempting to solder a new valve on. A basic knowledge of sweating pipes is needed for this project as well as basic plumbing tools like a tubing cutter, torch, abrasive cloth, flux and solder.

What you are noticing is the difference between pipe and tube sizes.

Nominal tube size refers to an exact measurement of the finished outside diameter. Nominal pipe historically referred to an exact measurement of the inside diameter of a particular type of iron pipe; however, it is the outside diameter that is important regarding fittings and, as pipe construction materials changed, the wall thickness and I.D. changed causing the I.D.to be different than the nominal pipe size of modern pipe. Roughly speaking, the actual O.D. of small pipes is around 3/8 inch larger than the nominal I.D. size.

Recent plumbing materials such as PEX are sold using tube sizes, so 3/4 inch PEX has an actual outside diameter of 3/4 inch, and a 3/4 inch quick connect valve for PEX has an actual I.D. of 3/4 inch. On the other hand, a nominal 1/2 inch brass pipe has an actual O.D. of 0.84 inches. A 3/4 inch tube valve will not fit on a brass 1/2 inch pipe.

The “85 red” label means the brass alloy used to make the pipe is 85% copper, which is de-zincification resistant. Early PEX fittings were made using brass that contained less copper and more than 15% zinc, and the zinc leached out of the brass causing the metal to corrode and fail prematurely, especially when used on hot water lines.

(BTW, common 3/4 inch copper water "pipe" is actually 7/8 copper tube.)