1. First create object of Package for this you have to include System.IO.Packaging
e.g.
Dim o_Package As Package

2. Open Package using open method of Package class its take three arguments
o_Package = Package.Open(sFilename, FileMode.Open, FileAccess.ReadWrite)

3. Create instance of XmlNamespaceManager
e.g .

Dim o_XMLNSMgr As XmlNamespaceManager = New XmlNamespaceManager(o_NameTbl As New NameTable)

4. Next Create instance of xmldocument class
e.g.

Dim o_XMLDoc As XmlDocument = New XmlDocument(o_NameTbl)

5. Create object of Package Part

e.g.

Dim o_PckgPart As PackagePart

Now using getPart method you can uas any Part of that Package it will take Parturi as argument

Using Package Part you can Modify Any object of PPTX like Graph, Text, Table etc. same way you can also modify XLSX and DOCX file on runtime.