How do I make my soldiers better?

Solution 1:

Soldier's stats get a boost based on their Morale. The Morale bar is the blue one next to the Diamond Dogs logo, above the health indicator:

Morale ranges from 0-15 and is reset every 36 minutes. Morale is tied strongly to your GMP balance, keep it positive to never lose Morale, Morale also steadily rises as you progress through the game, so long as GMP is positive. You can also raise Morale of soldiers by visiting them at Mother Base, just run up to a soldier so that he sees you and he will salute you, any method that you use to get their attention works, so long as they see you, only the soldier that saw you and saluted to you will get their morale raised, check which soldier it is by marking them (their name will appear above their head).

Source - this guide