Settings->media

Wordpress thumbnail settings page

Here you can change the width and height as you required. If you tick the option "Crop thumbnail to exact dimensions (normally thumbnails are proportional)" then the thumbnail size will be exact same as you specified. It will be a cropped image. If you remove this tick, Wordpress will take either width or height to generate the thumbnail.

Now, if there are many articles already posted with lot of images and you want to regenerate all the thumbnails. Is there any easy solution?

Yes. There is a smart plugin called "Regenerate Thumbnails". Regenerate Thumbnails allows you to regenerate the thumbnails for all of your image attachments. This is very handy if you've changed any of your thumbnail dimensions (via Settings -> Media) after previously uploading images.

Click here to download

After installing this plugin got to its setting page and click regenerate thumbnails. It will regenerate thumbnails of all the images uploaded previously.