This requires you to wrap a while loop around the input part of the program. The loop should terminate when the user types anything other than 'y' at the end of a record.
The prompting for input merely requires the addition of suitable printf statements.
The struct obviously needs fields for mean, standard deviation and an array of frequencies. It will also need to record the number of items read and the cumulative total of their values, since the mean is this total divided by the number of items. The standard deviation can be computed if you also keep the sum of the squares of the values, since the following formula gives you the standard deviation:
Square Root(n * (sum of squares of values - square of sum of values)/(n*(n-1)))
where n is the number of items read.
Back to the questions.