It's likely you are overwatering the plant. Trim off the brown leaves and repot. If there is standing water or excessive dampness in the bottom of the pot, where the roots reside, back off on watering for awhile, after dampening the new potting soil.
heterzygous with brown leaves
The color of the dominant trait will be expressed by that plant (Brown).
For some plants, the brown leaves slow the growing of the plant since brown leaves are basically dead leaves, but for most plants, people just cut them off because it looks more presentable that way. Usually they can live and grow with the brown leaves still there.
When the flower is dead, cut it off unless you are hybridizing. Let the leaves alone till they turn yellow and brown. Other than that, no pruning is needed.
Tobacco leaves
Due to leaf senescence. In autumn the leaves stop being the sink for phloem transport so the nutrient supply dwindles so the leaves age and turn brown (since there are no nutrients to carry on producing more chlorophyll etc.). Senescence occurs due to reduced levels of cytokinin (plant "hormone") production.
yellow/brown
The plant is either over fed, over watered, or given too much direct sunlight...or all of the above
The leaves help go through photosynthesis, which in turn makes energy for the plant. Your welcome!
Well, it depends on the season.
it is dying stupid
Yellow leaves usually indicates overwatering. Brown leaves indicate that the plant is not receiving enough water. The combination indicates that either your mint plant is confused or you are not consistently and/or correctly watering it, according to its need.