Replacement old date value with new date value

Question

Hi there, I hope your appreciated help.

First of all I must say that I am a newbie when it comes to net language.

The platform is C#.

This is my problem.

I need replace the value of DateTime.Now.ToString("dd/mm/yyyy") in all slides of my powerpoint presentation generate in OpenXml.

Now if try my net code the new value of DateTime.Now.ToString("dd/mm/yyyy") is superimposed in the old value of DateTime.Now.ToString("dd/mm/yyyy")

If you have link for similar task, please give it me.

Can you help me? thank you in advance.
Your help would be very appreciated

Thanks for your time and hints.