I would suggest repairing the valve as close quarters like that you may have problems.
To me that looks like a compression fitting if the valve can not be repaired.
With the water main off 2 wrenches are needed 1 to hold the nut and 1 for the valve.
Unscrew the nut from the valve and slip the valve off, It may take some back and forth twisting to remove.
I believe this is compression because I don’t see a sweat fitting or evidence of sweating. The nut facing the faucet is a classic setup for compression where a brass bead, band is compressed onto the copper the nut is trapped but a new compression valve can be attached to the nut
That's almost certainly a compression fitting valve. I've grown to dislike those "twist-in" style valves because they contain a rubber gasket that can corrode over time. They can also be a nightmare to open and close.
If you remove the old fitting (be sure the water is off first!), there will be the nut and compression ring. They should slide off easily once the fixture is removed). My personal preference is to use quarter-turn ball valves (easy to turn off and less prone to corrode in the shut-off parts). If this is the only one you want to replace, spend the extra money and buy a Sharkbite. They are a special type of compression seating that slides on without any tools. Just make sure the end of the pipe is smooth (can use a metal file on it if it's not).
As a final tip: buy a shiny new flange cover while you're at it. They're cheap and yours looks corroded.
Get a piggy back valve. You keep the existing one in place and just screw the new one onto the threaded end of the existing one.
If you want to just have one then you could shut the main water supply to the house off. Cut the existing one as close to the nut as possible - recip saw or osc tool or hand hacksaw. Sand and debur the copper and put a sharkbite push valve to replace it.
Beside other recomendations, do not unscrew the compression seal. They are meant to last life time and not to be broken. During the compression process, the compression ring will make a permanent grove in the copper pipe. Breaking that seal there is not guarantee it will not leak. Cutting it off from coper pipe and replacing with new, is only a option, only if you have enough space to the wall.
It might be something simple like loose handle, not closing the valve. Check the Philips screw and tighten it. To check, use a marker and put a dot on the white shaft. Do you see the dot moving when you operate the handle ?
Alternatively if the main seal is leaking:
Use Philips screwdriver and unscrew the handle.
Unscrew the nut around the shaft of the handle.
Under it you will find the first seal, used to prevent leaking to the outside.
Unscrew/pull out the valve guts.
At the end part there is a second seal.
That one is for water shutoff.
Replace both seals and assemble.
This is just to show what a "repair valve" or "piggyback valve" looks like:
Note that the female end has a loose captive nut and inside brass end designed to match the shape of a male compression connection. The captive nut gets tightened down to force the female end and male end together to make a solid metal-on-metal compression-quality seal without any ferrule or gasket or plumbers tape.
3/8-Inch O.D Female Compression x 3/8-Inch O.D Male Comp. A Lead-Free Brass Quarter Turn Water Shut Off Straight Repair Add On Stop Ball Valve,Chrome For Kitchen,Bathroom Faucet