I am a B.Com student. How should I improve my arithmetic & reasoning skills?

Hello, Yes, it is really important to improve your reasoning & quantitative aptitude skills. As it is asked in every competitive Exams. You should learn short tricks & formulas to solve questions easily & quickly. For this, you need to solve previous Year's question paper. Take help from R.S. Agg

You need to study basic math. For that, refer to RS Agrawal. Try to get enrolled in a coaching institute for reasoning.