Replacement Function

When I make work sheet and I make name column , phone. Mobile ......etc

When I write in Names column pater . Toni...etc.

Instead to write Peter every time I write only the number 2 for example
press this link to see example
\1

When I make replacement for number 2 to write peter the

I make replacement from :-menu edit – search – replace

whole sheet carry number 2 will change to peter

My Question:- how can I change only the number 2 in name column only without
Change the whole sheet.

Which function can I used , can I programmed it?

second Point::
In the excel cell this have 7 number :

For example

5741896
5287682
5852413
5147849
5174589

i want to replacement the number 5 in left side with 550/ only the 5 in beginning left side in same cell .

the problem is any 5 in the same cell is replacement by 550/
which function can i use it ? can i programmed it?