How to do a Calvary enhancement under technology research?

Enhancement is an action that you can do for each troop type and must be unlocked for each by doing research. Check in the Institute research options for the research tract for each troop type.

Under the Cavalry Training research tract you will find Cavalry Enhancement about half way down. Once you reach and complete this research you can then enhance any cavalry troops you have.

To enhance them tap on the Military University building and select the troops option to find all of your troops listed. Tap on the cavalry troops items and tap the enhancement icon (arm) below it to start the enhancement process.