// Load presentation
Presentation pres = new Presentation("mytextone.pptx");

String strToFind = "search string";
String strToReplaceWith = "replace string";

// Loop through each slide
for (ISlide slide : pres.getSlides()) {
	// Get all text frames in the slide
	ITextFrame[] tf = SlideUtil.getAllTextBoxes(slide);

	for (int i = 0; i < tf.length; i++)

		for (IParagraph para : tf[i].getParagraphs())

			for (IPortion port : para.getPortions())

				// Find text to be replaced
				if (port.getText().contains(strToFind)) {
					// Replace exisitng text with the new text
					String str = port.getText();
					int idx = str.indexOf(strToFind);
					String strStartText = str.substring(0, idx);
					String strEndText = str.substring(idx + strToFind.length(),
							str.length() - 1 - (idx + strToFind.length() - 1));
					port.setText(strStartText + strToReplaceWith + strEndText);

				}
}

// Save the presentation
pres.save("myTextOneAspose.pptx", SaveFormat.Pptx);