UNIX for Dummies Questions & Answers

Ok, I am brand new to UNIX and I am trying to learn a cross between basic script and database use. I had got some ideas off the net on simple ideas for learning UNIX. I am working on creating a simple phone book program that allows myself to enter our employees from work into a phone book text file. I got this idea from school syllabus online. I didn't know it would be so hard. Anyways I made a basic menu, can remove names from the text file, can add names to the text file, but can not for the life of me figure out how to change one if I type it in wrong or a name change or something. When I add the user records it is like as follows:
$name:$address:$city:$state:$zipcode:$phone

The change program in short is as follows:
# I enter the name (George) and a record or records show up.
# I select the by number which is assigned by grep
grep -in "$name" phonebook|sed -n-e 's/^/\
/' -e 's:/\
/gp'
#now George is selected by a number record...ie...3
# and George is stored as a record in this program.
echo "Enter the record you want to change: \c"
read record
echo "You have selected the following record: \c"
sed -n -e "$record"p phonebook:sed -n -e 's/^/\
/' -e 's/:/\
/gp'
file=`sed -n "$record"p phonebook`
name=`echo $file | cut -d: -f1`
address=`sed $file | cut -d: -f2`
# I do the same for all of fields.. city, state, zip, phone...
#then I prompt the user if they would like to change the name, #address..etc if yes then the following code is in a if statment.
if [ "$ans" = "y" ]
then
echo "Enter new name: \c"
read newname
name=newname
fi

now I am not sure how to get it to over write the old record or create a new one and remove the old record.

Thanks
George