As soon as you spot a cracked tyre on your car, ideally, you should replace it with your car's spare tyre. Then, you should drive to a garage to replace it with a new tyre, making sure that you replace it with the most appropriate tyre for your car. Unfortunately, cracked tyres cannot be patched up, making the purchase of a new tyre absolutely necessary.
It may be tempting to delay the purchase of a new tyre because of this. But remember that you are taking a big risk by not replacing damaged tyres right away.