Put a standard screwdriver head in the notch. Gently tappity tap tap the head of the screwdriver with a hammer. You want to go counter clockwise. It’ll loosen up nicely and turn right out. More on reddit.com

If you are in a condo, they may require a licensed and insured plumber to do this. Don't do it on your own if that is the case. You open yourself up to all sorts of liability. If it has stops on the valve, you should be good to go. Pop the plastic cap is the front of the knob off. Take out the screw behind it. Don't drop it down the drain. Take the two screws out of the cover plate. Pull the trim off, and shut off the stops. No stops means you need to shut the water off to the house. There is a bonnet nut holding the cartridge in. You may be able to take it off by hand by holding in the cartridge. You may need water pump pliers (channel locks). Take the cartridge out and bring it to a plumbing supply house. They should have the cartridge and point you in the direction for trim. If it is the valve I think it is, you will be very limited on trim, but they still do make a few options. If any of this flummoxes you, call a plumber. If it is the 600 series, and the bonnet nut doesn't want to turn, don't force it. There are three tubes in back that can twist and shear off. If it won't come off easily, you will probably need a new plumber and new shower valve. More on reddit.com

It’s generally a good idea to take the local plumber’s advice on brand, as he is more familiar with what’s used in your area. For example, all these guys are saying Moen, but we aren’t a big Moen territory where I live. Most plumbers know Delta and Kohler. If you buy Kohler, you need to buy an 8304-KS-NA. This is the valve that 90% of their shower trims are made to fit on and it has stops (which allow it to be serviced without turning water off to your home). This one has sweat/threaded connections that allow the plumber to use any piping system on it. 8304 trims have one handle that goes left and right to control volume and temperature at the same time. If you get Delta, buy an R10000-UNWS. This is Delta’s universal shower valve, it works with all of Delta’s shower trims, most of Peerless’ shower trims (Delta’s builder grade brand) and most of Brizo’s trims (Delta’s higher end, showroom brand). Delta has three trim types: T14, T17, T17T. The T14 has one handle that goes left and right, and controls temperature and volume together. Both the T17 and T17T have two stacked handles that turn left and right, the smaller one in front controls the temperature while the larger one in back controls volume. The T17T is the same function but with a different type of cartridge that allows for a higher flow rate. If you get Moen, you have two options, Posi-Temp and M-Core. Posi-Temp: Get 2590 valve (threaded connections that can be adapted to any pipe, with stops) Posi-Temp trim has a single handle that controls volume and temperature together by going left and right. This valve will work with any Posi-Temp trim. M-Core: Get the U140CIS valve (copper sweat/threaded connections that can be used with any pipe, with stops). M-Core has three trim types like Delta does, the 2-Series, 3-Series, and 4-Series. The 2-Series is like the Posi-Temp, Delta T14 series, and Kohler, it has one handle that goes left and right to control volume and temperature at the same time. The 3-Series has one handle that pulls out to control volume and goes left and right to control temperature. The 4-Series is like the Delta T17T series, it has the same type of higher flow cartridge and uses two handles, one small to control temperature and on large to control volume. This valve will work with any Moen 2,3, or 4 series trim. My suggestion is to find a plumbing supply house/showroom local to you and tell them that you need to buy a valve, and explain your situation. They’ll know what local plumbers prefer and help you make a selection. Hope this dissertation helps. More on reddit.com

Well, Delta has a reliable shower valve. The cartridge can be swapped out with success multiple every few years and that extends the life of the valve. Delta has multiple trims, you can convert between single and two-handle, it has integrated pressure balance and temperature control. Delta isn’t perfect, but they’re by far the easiest to service, repair, and install, and their customer service actually responds, which is pretty big for me (service tech.) By comparison Moen is cheaper and prone to breaking, seizing up. Kohler is over-complicated and overpriced. If you got Tuscany or Pfister or another junk brand then your contractor did you a solid. More on reddit.com