Some mechanics would tell you that the camshaft housing has to be removed (together with the camshaft and all that this implies, timing belt, etc.) to change the adjusters. This is not true, nothing more is needed than removing the camshaft cover and, naturally, knowing what to do.
It is difficult to figure out which one of the adjusters is dead. At most you can narrow it down to one cylinder. Take off the intake manifold extension part (black plastic or metal if fitted with EGR valve) by removing the hose from the intercooler and unscrewing three bolts in the back (two are under the plastic fuel hose holder, just pull them off), and unscrewing two smaller screws between the intake pipes of cylinders 1-2 and 3-4, close to the front edge of the head. If you run the engine at idle with the manifold extension off, you will hear a popping sound from the inlet of the cylinder whose adjuster fails. It is sometimes difficult to know which one it is, find a long plastic pipe approximately the diameter of the inlet on the head and put it onto each of the inlets in turn. The absence of the popping will be obvious when you put it over the inlet of a cylinder with the bad adjuster(s).
When you have localized the bad adjuster, stop the engine and open the camshaft cover. Wipe the gasket clean, it doesn't need to be replaced. You will see the rockers under the camshaft. Use something blunt that does not shed any particles (like a wooden spoon or a piece of plastic) to press on the side opposite the valve, the adjuster is under the rocker on that side. If you can press any of the adjusters down easily, they have either failed or will fail in the near future (the one-way ball valve in the adjuster normally makes it very difficult to press the adjuster down without applying a lot of force). Even if only one cylinder made the popping noise, other adjusters (not seldom all of them) can be already soft as well. The best way would be to change all of them as the rest will follow the bad one in due course but at least change all adjusters on the cylinder which was found generating the noise.
To replace an adjuster, turn the engine into a position where the inlet valves are open and the exhaust ones are closed (turning the engine is more difficult with an automatic gearbox, with a manual one you can simply put it into 5th gear and move the car a little bit). Press onto an exhaust valve until it moves down, push the rocker from the valve side towards the adjuster. Depending on the tools you have, you may even pull the rocker out by its wider part that goes over the adjuster. It will be a tricky operation, you might want to have a helper pushing down on the valve while you get them out grasping them on both sides of the rocker with your fingers (doing all this on a cool engine will be much more comfortable...).
Once you have removed the exhaust rockers, move the engine a little bit further until the inlet valves close and repeat the procedure for the inlet rockers. You will find that it is easier to knock the rockers off the adjusters by pushing them towards the center where the exhaust rocker were. Be careful not to lose the guiding parts on the top of the valve spring assemblies. As soon as all rockers are removed, the old adjusters can be pulled out easily.
Pour fresh oil into their hole and insert the new adjuster straight out of the package without wiping off the oil or squeezing them or anything. The rockers go back by carefully locating them (intake goes first), valve end first, until they sit on the valve, then pushing them over the adjuster. Move the engine back again until the exhaust valve cam is facing up, press on the valve and insert the rocker.
Close the rocker cover but before replacing the manifold extension, listen for the popping noise once more. It is normal to have some noise for the first seconds, it might even require to race the engine for a very short time to drive an air bubble out.