If it comes to it, how easy are these valves to fix or replace?

[ElCI]

Apologies as this is a bit of a cross post. I've had to try to isolate my washing machine hose tonight for it to be replaced tomorrow night.
When going to do this I noticed that the valve wasn't fully open/vertical. I've tried to close it and can't get it fully horizontal. Had a bit of a Google search and seen comments similar saying that it indicated the valve was faulty and when re-opening it started to leak.

Looking at the 1st picture below it looks like it might already have been taped up from a previous leak?
My question is, should it start to leak tomorrow night when re-opening, what are the options? Can it generally be fixed (even in the short term to get me through to next week)? And if not, how difficult are they to replace?

 

[Mottie]

Very easy to replace. Turn off the water at the mains, loosen the nut, fit a new one using the existing nut and olive if they don't come off easy.

 

[Exedon]

Easy enough to change turn water off adjustable spanner pair of grips 2 minutes job .
Replacement less than £2 not worth messing with

 

[Madrab]

If re-using the nut and olive - no reason not to as long as the olive isn't misshapen - buy a roll of Gas PTFE tape and put a wrap of tape around the olive, minimises the risk of leaks.