What do you mean "upgrade my tile yields"? All you have to do is move the builder (either a captured one or one you've generated) to the tile where you are building a district. When the builder is standing on that tile, one of the builder action items you can select is to boost production of the district. Using the builder to improve other tiles may indirectly help with district production, by boosting the overall production of your city (depending on what tiles you've improved), but that is not what the Aztec ability is all about.
As an aside, when playing as China, which can use builders to hurry production fo ancient and classical era wonders, you do the same thing -- move the builder onto the tile where the wonder is under construction and select the wonder production builder action.