As long as you don't activate it (by passing the reader over it), you can apply it when you like. Many people apply it 24 hours before the old one ends so as to let it "bed in" before activation (they're often less accurate during the first 24 hours).
As long as you don't activate it (by passing the reader over it), you can apply it when you like. Many people apply it 24 hours before the old one ends so as to let it "bed in" before activation (they're often less accurate during the first 24 hours).