How to change the spatial reference in a polygon feature class?

[Colcam4]

I have feature classes from 3 different Spatial Reference Projected Coordinates that should all be the same:   NAD 1983 (2011) UTM Zone 14N_. How can I make them all the same without loosing or corrupting data?

1.) NAD 1983 (2011) UTM Zone 14N_

2.) NAD_1983_UTM_Zone_15N

3.) NAD 1983

[JayantaPoddar]

You could reproject the data using Project (Data Management) 

[Colcam4]

How does that tool affect your data, and will a New Feature Class be made, and if so, what do you do with the old one that has the original name you want to keep?

[JayantaPoddar]

For reprojecting your data from one coordinate system to another, You could use Project (Data Management) . It will create a new output. If the data overlays properly, as per your need, you may delete the older file, and rename this file with the original name.

*Check the step below only if the above step doesn't work. 

In case the coordinate system is incorrectly defined for the dataset, you need to use Define Projection (Data Management). This will change the properties of the original file. Only execute this tool if the defined coordinate system is incorrect as per the units and extent of the data.

[JoeBorgione]

You may also want to check out this: https://learn.arcgis.com/en/projects/choose-the-right-projection/

Understanding projections, spheroids etc is a good thing to know.  

[Colcam4]

Thank you all for this insight. Very much appreciated.