Hello, I don’t understand how I should do to revert to an earlier version of a blender file. I have instructed blender in the user preferences to save 5 versions of my project but where does Blender save them?
The verious versions are saved in the same place as the original and can be identified by the number apended to the file extension.
.blend - active version
.blend1 - Previous version
.blend2 - next older version
and so on up to .blend9
To open an older version of your file simply change the file extension back to [.blend]. You should probably name it something else while you’re at it 
Sorry, but when I try to save my file with extensions like filename.blend1 etc the file is saved as filename.blend1.blend.
What am I doing wrong?
What I usually do is filename-1, because Blender will add .blend automatically. Then when you want to save a second version, you can name it filename-2, and again Blender will add .blend automatically.
The .blend1, .blend2, and so on that @Capricas_Kirito was talking about is what Blender will add automatically every time you save the same file WITHOUT changing the name. IOW, if you originally name the file “filename”, and then make some changes and save it again without changing the filename, Blender will save them as .blend1, .blend2, and so on, leaving the most recent as just .blend. The higher the number at the end indicates the oldest saved version of the file with the exact same filename.
the .blend1 is an automatic change that the program makes when loading a file. Anytime you save something the program will automatically append the .blend extension unless the name ends with it already. Thus, naming a file [Workflow.blend] is the same as naming it [Workflow] because blender will automatically append the name so that [Workflow] becomes [Workflow.blend]. However it will read [Workflow.blend] as a blender file named [Workflow].
Thus, naming a file [Workflow.blend1] results in the program appending the [.blend] to denote it as a blender file named [Workflow.blend1]. To avoid this conflict consider naming the file [Workflow_1] or [Workflow_Backup]. The important thing is consistancy within your naming structure so that when you look at the file name you don’t have to wonder what it is.
Thank you Capricas and Miss_B for trying to help. I believed from the start of this discussion that the the .blend1 .blend2 and so on where added automatically by Blender in just the way that you two describe but was not sure so thanks for clarifying the matter. The issue here is that I was not sure and the reason for that is that I can’t still find this .blend1, .blend2 … files in the directory where I save the stuff. I can only find the first version named filename.blend. I have (and checked multiple times) modified the user preferences under file so that I should save 5 versions (the slider you know). But still I can’t find those files. 
Out of curiosity, how are you trying to find the files, through blender or through File Explorer?
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Through Blender, I just checked with File Explorer and then I can see the .blend1, .blend2 etc.
Thanks
If you go into your User Preferences you will find a tick box on the File tab that allows you to show/hide the extra versions.