Sometimes you cannot afford changing all four tires together you may just have to replace tires as you can afford them, one or two at a time, but there are some important things to think about before doing that.
It’s essential that you go with tires that are identical to the car’s remaining tires. That means that internal construction, size, tread pattern and design should be close to the same.
Don’t mix winter tires with all-season tires, don’t mix run-flat tires with standard tires, for instance.