The below code gets the default template library and active template. Collects all the point elements and add prefix to point having name "BAR_UG2" .User can add or edit the name as required.

//Required References
using System.Xml;

 public void AddPrefixOrSuffixToPointName()
        {

            //Prefix to add to point name 
            string prefix = "PREFIX_";

            //Get default template library
            string strTemplateLibraryPath = Bentley.CifNET.GeometryModel.SDK.TemplateLibrary.GetDefaultTemplateLibraryPath();

            //Load the Template library as XMLDocument
            System.Xml.XmlDocument doc = new System.Xml.XmlDocument();
            doc.Load(strTemplateLibraryPath);

            //Get all "Point" node from template library
            XmlNodeList pointsList = doc.GetElementsByTagName("Point");

            //Read each point to add prefix or suffix, here for demo purpose first point is used
            foreach (XmlNode pointNode in pointsList)
            {
                XmlElement pointEle = pointNode as XmlElement;
                if (pointEle != null)
                {
                    //Get point name
                    string pointNameAttribute = pointEle.GetAttribute("name");

                    //Sample point BAR_UG2 is used here to add prefix
                    if (pointNameAttribute == "BAR_UG2")
                    {
                        pointEle.SetAttribute("name", prefix + pointNameAttribute);
                        break;
                    }
                }
            }

            //Save the template library document
            doc.Save(strTemplateLibraryPath);

        }

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Output