You have to reference the slide that the object is on and the content object name such as "Title 1". Example is below with attached "myPres.pptx".
In PowerPoint, you can see the content object names by going to Home>Select>Selection Pane. For example on a basic title page you might have "Title 1" and "Subtitle 2". If you know what these are you can refer to them directly:
import mlreportgen.ppt.*;
pptfile = 'myPres';
pres = Presentation(pptfile,pptfile);
replace(pres,'Title 1','New Title Using Option 1')
close(pres);
clear
If you do not already know the name, but know how the object is referenced as a child of the slide, you can access it through MATLAB:
import mlreportgen.ppt.*;
pptfile = 'myPres';
pres = Presentation(pptfile,pptfile);
s = find(pres,'');
objName = s(1).Children(1).Name;
replace(pres,objName,'New Title Using Option 2')
close(pres);
clear
For more information see the below documentation pages: