A smooth running chain is something that we would hardly take notice of, but as soon as it slips, crunches and drops gears when you are out of the saddle, accelerating away or merely leaving a set of traffic lights it's a real annoyance and becomes the bane of a good bike ride.
As the chain wears, it deforms the sprocket teeth, which in turn leads to mis-shifting. Dirt and grit cause fricton that also wears the rollers between the links.
Therefore it is best to replace your chain just before it gets completely worn. In doing so, you'll prolong the life of your cassette and chain rings. If you are unsure if your chain is worn, you can buy a specialist tool to detect wear. Alternatively, measure 12 complete links. If the chain is 308mm or longer, your chain is worn out and should be replaced along with the cassette.
Here is the Condor four step guide to fitting a new chain.
SHOP THE TIP: Condor Chain Tool | Lezyne 11 Speed Chain Tool | Park Chain Wear Indicator
SHOP THE TIP: Condor Chain Oil for Winter | Muc-Off Chain Oil for Summer