Like you access any other wc3 resource. Opening the standard mpqs in Wc3's main directory with an mpq editor. Wc3 loads and merges those mpqs at startup, the precedence being War3Patch > War3x > war3, meaning if there is a file with the same path in War3Patch, it overshadows the one from War3x/war3.
Most of the slks/txts are in Units\
The human proper names assignments e.g. are in Units\HumanUnitStrings.txt
If the unit in your case is a custom object and you want the proper name to be different from the unit you based it on (if it is not sufficient to change the original unit's entry), then you would have to have the custom object be transformed into a new original unit altogether. There are several slks for units where you would have to enter it.