I apologize: I meant to say healthy basil

Is the brown at the base of the stems? And is it soft and brown or more as if it is turning woody? Basil does get woody and brown at the base and that is perfectly normal. If the brown is soft it is more likely a fungus. As to the leaf yellowing it is a matter of degree. No plant can hang on to leaves for ever and there is a natural turnover with new leaves coming and old leaves dying. But if there are a lot and it's happening suddenly it could indicate a problem. The commonest is over watering. When you say the drain holes are at the sides does that mean there are no holes on the bottom of the pot? If so there could be stagnant water sitting in the bottom.

Thank you for your reply. I have placed fresh soil in the pot , and I will stop watering it every other day. The drainage holes are on the bottom of the pot. I took another look at it and I figure the browning feels rather smooth and is definitely more of a light woody color.

Perhaps it is a combination of over watering and wooding? I did not suspect wooding as the plant is not that old , maybe 2 and half months old. The flowers are mostly white , but some of them are browning . There are very few yellow leaves.Much of them are green , but I have another basil plant that is a little younger and flowering less with less leaves but overall bigger leaf size. I haven't seen any seeds either.

Another (Note:I have placed soil AFTER I took this pictures)

Here is another

Looks like totally normal ageing of the stems. But you need to trim off the seed/flower heads if you want it to keep producing large fresh leaves. I would also fill the container to the top as it looks half empty at the moment and the basil will develop a big root mass.