I am a beginner in running who run about 2 miles per day. How can I improve it? And also I want to know, is that better to run in the same speed or to shift gear during the running?
In general, run training takes several forms, often considered as Strength and Endurance.
This can be accomplished with building running specific muscles, usually by hard running.
You run up a hill, pushing hard, to build the muscles you use when running up a hill.
This is often done on a track, running repeats of a specific distance. 100, 200, 400, 800 meter repeats. (Outdoor tracks are very often 400m or 440 yards in length. So 200m is half a lap, 800m is two laps).
The problem for you, is that 2 miles at a time is not quite enough of a base of running to really start in on these training methods since you almost certainly will get injured due to lack of fitness.
This can be thought of as people training for longer distance runs, like a half marathon (13.1m/21.1k), marathon (26.2m/42.2k) or even longer.
I suspect, if your base is 2 miles at a time, you need to focus on endurance first.
Sadly the basic way to do it, is simple and almost tautological. To run longer, run longer.
That is, if you can do 2 miles a day, then try one day to run 2.5 miles, while keeping the other days at 2 miles. Then keep increasing one or two days a week. (Space the longer days out as far apart as you can to allow for recovery, since the harder workout breaks down muscle, and it grows back stronger, which is how you get stronger, but all that takes time to happen. So recovery is key).