Every unit graphics are implemented in the graphics.drs file in your data folder.

To open that file you need a tool like Turtle Pack.
Each entry in the graphics.drs is an slp file, containing several bitmaps.
Each unit has 5 or more animations each beeing represented by such an slp file.

You can extract the slp files using Turtle Pack from the graphics.drs and also the bmps.
After changing the bmps, you have to convert them to the AoE color palette (this also can be done with Turtle Pack).
Now you can replace the bmps contained in the slp file.

For replacing the slp file, you can replace the one contained in the graphic.drs or rename the slp from its ID to it LongName, that can be found in the games dat file for example.
If you want to rename the slp, just put it in your slp folder and if you named it correctly it will replace that file out of the graphics.drs automatically.

If you want to replce a unit, think of the player color, as that has to have specific ones out of the palette so the game can recognize it correctly.

If you have further questions, don't hesitate to ask.^^ Hope this helps if it should be needed anymore.