Look up the most recent exchange rate for Swiss Francs to British Pounds. The exchange rate can be found in newspapers or online through a search engine.
Multiply the number of Francs you have by the exchange rate. For example, the exchange rate on July 16th, 2010 was 0.6240. A man with 100 Francs would multiply 100 by 0.6240 and obtain 62.40. Therefore, he would have the equivalent to 62.40 GBP.
Locate a currency exchange counter or bank to exchange your Francs for GBP. Some establishments charge a fee for converting currency, so compare the exchange rates and fees of various locations before trading your Francs.