The Tiefling has the Infernal Legacy racial trait, which grants them the Thaumaturgy cantrip.
In the PHB, it says (emphasis mine):
[...] when you gain a level in this class, you can choose one of the warlock spells you know and replace it with another spell from the warlock spell list, which also must be of a level for which you have spell slots.
Suppose I have a Tiefling Warlock that has just reached level 2. Can I replace the Tiefling's Thaumaturgy cantrip (from their racial trait) with another Warlock cantrip? Or can I only replace specifically Warlock spells of level 1 and higher?