One way is to connect the Viewer Node.
Then just open your Image Editor, select Viewer Node and save the image.
You can save to any format you need .jpg/.EXR are all supported.
If the Viewer node doesn't show any image, just toggle one of your nodes off/on.
Viewer/Composite nodes refresh only when there's a change in the compositor.
There are a couple ways to interpret your question.
If you mean save out your render before the composite; go to your compositor then 'shift+a > output > file output' and connect it directly to your render output node.
If you mean that you rendered the image and are compositing using the live view after: There isn't an instant way of doing that presently. You can maximize the node editor using ctrl+space and do a screenshot or re render the frame. If its just for a preview image that you want to save you can turn down your resolution or ray count (be sure to have denoise in your composite node tree if you do that) to make it faster if time is an issue. The back image in the compositor is just a preview, it is not meant to be a final render product.
The way I save any scene is to use the print screen button on the keyboard. Click it then open up a picture editor like Microsoft paint because it loads fast, and paste, crop picture if you want, and then click save as. For more options on editing the picture use a program like Gimp to past the picture into.