You'll need to get a sponge or microfibre cloth and some fabric cleaner. You can use products that you might have kicking about the house - like vinegar and water or baking soda (which will dry the area and remove the smell) - but a proper cleaner will do a better job and it's easy enough to collect from somewhere like Halfords or even Tesco. Spray on a little bit of the cleaning liquid at a time and gently rub it with the cloth, don't rub it too hard or you can damage the seat fabric. Also, don't pour/spray too much fabric cleaner on at once because if you soak the seat and it doesn't dry properly, it can lead to mildew and it'll smell.

If you want to go for a more expensive, but probably more effective solution, you can hire a steam cleaning kit from various places if you live in a city (we recently hired a steam cleaner to remove spilt paint from a car and it's very easy and cheaper than buying one). If it's a big stain and/or it's started to smell like spoilt milk then this is likely a better option.