new_frequency(1:rowRPM(1,1)) = str2double(frequency(1:rowRPM(1,1)));
In the special case where you are converting all of the entries, then you can use
frequency = str2double(frequency);
but if rowRPM(1,1) is not equal to the number of elements in frequency, then you need to write to a new location.
In particular, you cannot work the way you are doing now, converting an entry and writing it back to the same array. The string() class will detect that you are storing a number in a string location, and will helpfully format the number as a string and store the string.