Keep all e-liquids away from direct sunlight and extreme temperatures.
Store them upright where possible with the cap secured.
Please note:
Sunlight can cause nicotine to turn darker in colour but this is natural and does not mean it is unsafe to vape.
Extreme heat or cold can change the viscosity of your e-liquid which can affect its performance in certain vape device types. It can also cause splitting which will make the e-liquid unpleasant to use as the nicotine concentration will become inconsistent.
Over time all e-liquids slowly steep - this is the process by which the flavour matures and changes. Steeping is not a bad thing over short periods, but very old e-liquids can over steep and become unpleasant to consume.