how to reduce the error in position by Odom?

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There are many methods to improve odom.

1 - Tune your odom - make direct measurements of encoder counts vs movement and rotation and use that data to improve the accuracy

2 - Optimize wheels for the environment - reduce slippage and error by using wheels appropriate for the environment - use narrow hard wheel on a flat smooth surface etc

3 - Optimize weight distribution - place the CM in a location that makes any slip when turning low and repeatable - the shape of your robot, wheels, rolling surface will determine that.

4 - someone else can list 3 more way to improve odom

With odom optimized, there will still be drift. You will not get away from it. Then other methods to mitigate the error are used usually referencing fixed environment like

1 - AMCL

2 - GPS

3 - land marks, etc.