How to replace specific data points

Lets say we have the following data set 
Attribute: Age, Gender, Floor 
 Row 1.    11     Male      20
 Row 2     11     Female  29
 Row 3     13     Male      18
 Row 4     12     Male.     17

 How do we change only the age of row 2 from 11 to 12 without changing the age value of row 1 from 11 to 12? Thank You