I’m 20 years old, 5”7 and weight 120 pounds, I was always underweight growing

Up but in grade 10 gained weight and was 190 at my biggest at 18 years old, I lost all my weight while at college and now am back to being small. I feel as if I eat a regular amount of food daily but can’t seem to keep any weight on, if anyone could give me any tips as to gaining weight I would appreciate it as I would like to do it a healthy way