Yes, you can rename voice clones, which in turn will be reflected in the custom voices selection menu.
To rename a voice clone:
Navigate to the left hand menu of Synthesia and under the Assets section, select voices.
Locate the voice clone you would like to rename and select the three dots in the top right corner.
Select Rename.
Enter your desired name and select Rename to save.
Notes:
To learn how to create a voice clone, see our How can I clone my voice? article.