You can't, they are an annual so you will have to buy new plants next year.

'Surfinias', like the older 'Supertunias' and 'Waves', are grown commercially from softwood cuttings, not seed. In the UK, it would be necessary to keep a large plant or two alive in a heated greenhouse over the winter, and take cuttings sometime in late winter, to be hardened off in the spring.