replace text

[C# Programming]

Hi
I have need a program for project
I project is:
text=abokf.okhj.jkoklk.
start=ok
end=.
replace=D

answer is:abokD.okD.jkokD.

please give me code http://www.barakasoft.com/script/Forums/Images/smiley_doh.gif

pleas hellp me
c#