You can increase the HP of your units through several methods, including:

-Adjusting the attributes of some passive skills for military officer skills as Li Wen Zhang and Lu Xianyue.
-Advancing tanks through the Logistics Center building.
-Increasing VIP level.
-Conducting technology research that is related to related to units HP at the research institute.
-Installing HP widgets in your equipment.

And other methods that you can explore as you delve deeper into the game.