Your sparten's stats can be improved by taking classes, the following list shows most of the classes in the game (credit to the spartan warfare 'game guide')
Course Basic Combat (2days) $200 stat gain: 50
Advanced Combat $400 stat gain: 75
Small Weapon Training (5days) $800 stat gain: 90
Assault Weapon Training $1600 stat gain: 140
Heavy Artillery Training $4000 stat gain: 200
Vehicle Operation Training (7days) $8000 stat gain: 300
Strategic Education $20000 stat gain: 1k
Stealth Training (2days) $80000 stat gain: 2k
Assassin Training(7days) $100000 stat gain: 2.5k
Special Ops (7days) $100000 stat gain: 2.5k
Modern Tech Certificate (10days) $100000 stat gain: 3.5k
Advanced Warfare Training (7days) $150000 stat gain: 4k
Battlefield Assessment Training (10days) $150000 stat gain: 4.5k
Leadership Training (10days) $250000 stat gain: 6k
Battlefield Stress Techniques (15days) $250000 stat gain: 7.5k
Alien Physiology (15days) $500000 stat gain: 10k
**Elite Ops Training (30days) $1000000 stat gain: 15k(10k intel)
Tactical Ops Training (30days) $1000000 stat gain: 25k
Hitman Basics (5days) $2000000 stat gain: 40k(not intel)
Hitman Apprentice (5days) $2000000 stat gain: 40k(not intel)
Hitman Master (5days) $2000000 stat gain: 40k(not intel)
Computer Hacking Course (30 days) $3000000 stat gain: 50k(intel only)
Black Ops Extreme (15days) $5000000 stat gain: 100k(not intel)
Spartan Intelligence (15days) $5000000 stat gain: 100k(not intel)
Spartan Intelligence Mastered (15days) $5000000 stat gain: 100k(not intel)
Spartan Elite Ops (15days) $10000000 stat gain: 250k (not intel)