please try using the SetObjectPropery-Call:
Change properties of objects:
This function can change most string and number properties of objects (names, numerical values, OIDs, etc.) and should be used with caution. The "property_name" parameter can be discerned by opening the "Settings" page of an object and looking at the HTML source of the INPUT fields. For example the INPUT field for the tags of an object has the name "tags_". Leave away the "_" and use the rest ("tags") as value for the "property_name" parameter.
Please also see your very own PRTG, under "Setup"->"PRTG API"->"Object Manipulation".