methun:x:500:500:comment:/home/methun:bin/bash
salahuddin:x:501:500:comment:/home/methun:bin/bash

echo "Enter a Name:"
   read username
   users='cat /mypractice/myfiles/testfile | awk -F ':' '{print $1}''
   for user in $users
    do
      if [ "$user" == "$username" ]; then
        echo "Name found and Enter a new name to change."
        read newUsername
          #need code to change text on my file --->testfile
      fi
    done

sed -i 's/"$user"/"$newuser"/g' /mypractice/myfiles/testfile

sed -i "s/^$user/$newuser/" /mypractice/myfiles/testfile