Below are a few ways to change signal names in the Scope legend. These are explained in the "Using the PLECS Scope" section of the PLECS Documentation.
Click the Cursors icon in the Scope window. The icon looks like two vertical bars with a zig-zag line through the middle. Once the cursors are activated, the data window appears if it was not already open. The signal names are displayed and can be modified by double-clicking on the name.
You can show a legend on top of each plot by choosing Show legend from the View menu or from the context menu. To modify the text, double-click on a label. If you erase the complete label text, the default text is restored.
The default text of a signal label is derived from the name of the block from which the signal originates and possibly a probe signal name. You can also edit these names rather than changing the text in the scope window.