Hello Hassan. 

I think what you are trying to do is to reclassify a raster image, that is, you want to assign new values to the raster pixels.  If this is what you want, you may look at this video.  It uses ArcGIS.

Also, read through the help file attached.

https://www.youtube.com/watch?v=D7qMG-7jmUo

http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Using%20the%20ArcGIS%20Spatial%20Analyst%20toolbar%20to%20reclassify%20data

Hi Hassan,

Reclassifying a raster as suggested by Eric would do the trick.

But if you are working with a raster that has a huge number of values, an easier way would be to do a simple calculation using the raster calculator in whichever GIS software you are using.

Lets assume you have a raster called R1. Identify the maximum value in R1 which you will find in the raster properties metadata (usually accessible by right clicking the raster name). Lets assume this to be 255

Open the raster calculator and calculate the absolute difference between the raster max and individual pixel values like so:

abs(R1 - 255)

Save the output to a new raster.

Alternatively, you could also find the max value for your raster within the raster calculator itself during the calculation.

abs(R1 - max(R1))

Hello . As suggested you should first decide how many classes your new raster will have.. Will it be something like 0->High, 1-> Medium, 2-> Low. 

Then you should  decide what will be the ranges of your classes . You could just use one of the preconfigured methods (like fixed intervals etc) or make custom ranges that suit your needs..

Dear Researchers,

Thanks a lot for all helpful comments. I've resolved the issue using Raster calculator Function.

Cheers.