It depends on the country of your first and second doses of vaccine. Beat in mind your second dose should be the same type as your first. In England, the NHS would contact people to bring forward their appointments. You can currently get the 2nd dose 8 to 12 weeks after getting your 1st dose. You can also rearrange an appointment yourself. You can do this online at NHS website, or by calling 119. There are also walk-in jab sites available, which you can find by entering your postcode.