I'm not entirely sure what you mean by 'distance value seems like decimal'.
Decimal is not a unit of distance. So, you can't change decimal distance to distance km.
However, you can convert from one unit of measurement to the other. For example, you can convert from meters to km or miles to km, etc.
You need to first find out what the unit of measurement of your data is. If the distance is already in km, then you don't need any conversion. But if it is in other units, you will have to apply the appropriate conversion factors.
Assuming the distance values are in km, this means that small distances will be decimal values. If you don't want the decimal values then you need to convert the distances to other units, for example meters.
1km = 1000m. So, multiply your distance km data by 1000 to convert them to meters. If you do so, remember to indicate the new unit of measurement in all your visualizations. So, the Y axis of your visualization should be labelled Distance (m) to show the values are now in meters.
Hope this helps.