If it is a plain band the dimensions could be increased by simply stretching the metal. However the metal with which the ring is made is essential here, because while some metals like gold and platinum might be easily stretched, tungsten and tungsten-carbide alloys are extremely tough to resize. It is also more difficult to resize rings that have an intricate band using more than one metal in use. http://weddingbyu.ie/guide-to-choosing-the-perfect-diamond-ring/