1. You should say 降，it means lower 2.you can't say 我降肥 3.减小音量（you mean volume?）/降低声音/放下手 4.降 opposite 升 减opposite 增 example1 增大音量 example2 升温 conclusion: 减 have two meanings, one is to remove a part things from the total things. As you see, 肥 is a lot of weight, we should remove some weight from our whole body. The second meaning of 减 is the same as 降：it means lower. Understand? However, it's a little bit difficult to use them properly. I advice you to understand and remember the meanings above, then read a lot of example sentences of those two words, you will be able to use them well~

降 is in contrary with 升 ， degrade, lower, descend , they are talking about height and level...usually used this way 上升，下降，降低，升高 减 is in contrary with 增 reduce, less, minus, they are talking about quantity， usually used this way 减少， 增多 1) Please don't manually lower the air conditioner temperature to 20 degrees Celcius. It's better if it's set at automatic 22 degrees Celcius. 请不要把空调的温度手动降低到20度。设置在22度比较好。 2) I know people say, "我减肥“ can you say "我降肥“？ no. never say 降肥。 3）How to say lower the music/ lower your voice / lower your hands? 音乐声调小一点。 你的声音小一点。 把你的手放低一点。 4) 我要降火。Can I say "我要减火？” we say 上火 and 降火， never 减火。Our ancients think 火 is something going up.. 4) What is the opposite of 降 and 减？ How do you use them in the sentence like: a) Raise the volume, I can't hear anything. the volume of machine 声音调大一点，我什么都听不到。 the volume of human being 大点声，我什么都听不到。 b) Can you raise the temperature, it's hot and stuffy in here. raise? or lower? 温度可以调高点吗？这里又热又闷。

There are some phrases, and we don’t replace“降” or ”减” with each other in Chinese. For instance, there are 降低（reduce），降火（=always say 去火，is a Chinese medical expression ,it’s like Endocrine regulation），下降（fall vi.） etc；there are 减少（cut down），减轻（lighten vi.），减肥（lose weight） etc. All these phrases are used in proper conditions. 降低：温度（temperature）降低、（物价）水平（price level）降低、降低要求（requirement）； 下降：用于表示位置的变化；数量下降=减少； 减少：人员（amount of people）减少、麻烦（problems）减少、缺点（weakness）减少； 减轻：负担（burden）减轻、病情（state of your illness）减轻； 降火、减肥 don’t have much more usage.

I feel so bad for understanding that you are too busy to read these answers, so your best answer was voted by votes....It's really frustrating

两者意思相似，用法上搭配不同。例如：降---降低，减---减小、减少，后面跟的用作宾语的词也是不太一样的，可以看做习惯用法。