To convert a quantity from one unit to another, multiply or divide by the conversion factor. A smaller unit needs a bigger number to describe the same amount, so going from a big unit to a small unit you multiply. Going from a small unit to a big unit you divide.
Useful conversions: 1 km = 1000 m, 1 m = 100 cm, 1 m = 1000 mm and 1 kg = 1000 g. For time, 1 hour = 60 minutes and 1 minute = 60 seconds, so 1 hour = 3600 seconds.
Worked examples: 2.5 hours is 2.5 × 3600 = 9000 s. A mass of 450 g is 450 ÷ 1000 = 0.45 kg. A length of 3.2 km is 3.2 × 1000 = 3200 m. Going the other way, a film of 5400 s lasts 1.5 hours, a parcel of 2.5 kg weighs 2500 g, and an 800 m track is 0.8 km.
To change a speed in km/h into m/s, multiply by 1000 for the metres and divide by 3600 for the seconds. For example, 72 km/h becomes 72 × 1000 ÷ 3600 = 20 m/s. Always check that the size of your answer makes sense for the new unit.