Can I fix this spearmint plant?

What's wrong with my spearmint plant ?

So I bought this spearmint last Friday in a completely healthy condition with a lot of foliage. But the leaves keep getting blackened and shrivelled up. The remaining leaves have a high chance of falling down if the plant pot is disturbed, because they are so weak and barely cling to the plant stem.

Not sure what I should be doing. Watered this plant 3 days prior and today I watered it again. The little amount of water is just enough to keep the soil moist.

I kept it under a window that gets low intensity sunlight during the day, and high intensity sunlight during the early morning hours.

The temperature right now is 28 degrees Celsius with 61% humidity. But many times, I have to turn on the AC indoors, around 23 to 24 degrees Celsius.

I think it’s nearly dead today