Are the trouble spots on the lower leaves? They will eventually brown and fall off by themselves due to the aging process. Remove them if you don't like the look. The plant overall looks to be in great shape.

In tropical places they have to hack the plants back to keep them in check. Here is an example of what they can become if you lived in Maui (late March last year) Anyway you are doing great based on your photo of the entire plant.

might be the water you're giving it? are you giving it tap water? are you fertilizing the plant (if you are make sure you're not spraying the leaves with it)

Hi Hu. Your plant is a Dracaena fragrans. It looks like it has been grown in much lower light than it prefers. This puts it at risk for all kinds of infections & deficiencies. You can try moving it (slowly) to brighter sunlight, or add supplemental fluorescent or LED lighting. Once you do, the new leaves will be a healthier, lighter green, and have a more upright posture. Once in higher light, it will also appreciate frequent light fertilizing. I use Dyna-Gro's Foliage-Pro.