For this first you need to extract numeric part from the given value.
try to get numeric value than you can add particular number to that resulted nuber , after calculation again you need to add calculated number to string.
Follow this to get number part-
=1*MID(A1,MATCH(TRUE,ISNUMBER(1*MID(A1,ROW($1:$9),1)),0),COUNT(1*MID(A1,ROW($1:$9),1)))
refer this link-
http://office.microsoft.com/en-us/excel-help/extracting-numbers-from-alphanumeric-strings-HA001154901.aspx