Yes, there is a way to edit the legend text for a trendline in Microsoft Excel. Here's how you can do it:

1. Click on the chart to select it.
2. Click on the trendline to select it.
3. Right-click on the trendline and select "Format Trendline" from the context menu.
4. In the "Format Trendline" pane that appears on the right-hand side of the screen, click on the "Options" tab.
5. In the "Options" tab, you will see a field labeled "Custom Name". This is where you can edit the legend text for the trendline.
6. Simply type in the new legend text that you want to use for the trendline in the
   Code:

   "Custom Name"

   field.
7. Press "Enter" or click outside of the
   Code:

   "Custom Name"

   field to apply the new legend text to the trendline.

That's it! You have now edited the legend text for the trendline in your Excel chart without having to delete the trendline legend entry or create a "dummy" data series.