Cantrips don't use spell slots at all
Look back at the description of cantrips in the rules for spellcasting:
A cantrip is a spell that can be cast at will, without using a spell slot and without being prepared in advance. Repeated practice has fixed the spell in the caster's mind and infused the caster with the magic needed to produce the effect over and over. A cantrip's spell level is 0.
You don't use any spell slots to cast a cantrip; cantrips are effectively an unlimited resource.
Most cantrips do improve as the character gains levels - for instance, Eldritch Blast starts to produce additional attacks as you level up:
The spell creates more than one beam when you reach higher levels: two beams at 5th level, three beams at 11th level, and four beams at 17th level.
But this is related to your character level, not to spell level or spell slots. It's distinct from the concept of upcasting a "levelled" spell, where you use a higher level spell slot to produce a greater effect. Though a warlock has no choice but to upcast all their levelled spells at the highest level they can (since they don't have lesser spell slots to use), cantrips exist outside the spell slot system.