It is recommended to replace all ground contact tires at the same time. If you do not replace all ground contact tires at one time, then at least, replace all the tires on one axle.
If only one axle set of tires is replaced, then it is generally recommended to install the newest tires on the rear axle. This may complicate tire rotation, and caution is urged if the tires have significant differences, such as state of wear, tire size, construction, and speed ratings. Consult a trained tire specialist for best practices.
Placing greater grip on the rear axle is generally recommended to prevent a possible over steer condition and loss of vehicle stability on slippery surfaces. Some car manufacturers recommend to install the newest tires on the front axle. Consult with the car manufacturer and / or tire specialist for further information.