Nuttela needs a little help

Nuttela and horsbug were out fishing, when they suddenly remember the pending homework. They rush home to complete the task within a stipulated time. Provided with a string of length $N$, they have to find vowels in a particular sub string. Since, this would be very easy to execute, the teacher also asked them to update some characters of the string in between. Looking at the watch, Nuttela and ho