How to update Legend / series text of powerpoint chart using VSTO in C#?

hi Samantha,
It would be good if you can provide some code that you have tried so that the answer can be more specific, but as far as I understood your problem you can update the series or Legend item text by using its Name property for example the following code will update its text exactly as per your requirement. You just need to get your chart's SeriesCollection and update the Name for all items in the series collection.

var sc = chart.SeriesCollection();
                    int counter = 1;
                    int total = sc.Count + 1;
                    do
                    {
                        var seriesTemp = sc.Item((counter));
                        seriesTemp.Name = "my series" + " " + counter;
                        counter++;
                    }
                    while (counter != total);