The bad news is you can't soften watercolor paint in a tube once it's dried hard, to get it squeezing out like it used to. The good news is that this doesn't mean you can't use the paint.
What you need to do is cut open the tube to get at the paint, then use it like you would a pan or block of watercolor. That is, gently rub a wet brush onto the dried paint and it'll "dissolve" into the water. Try to fold over the edges of the tube so you don't have any sharp edges that'll damage the hairs on a brush.
If the paint has thickened but can still be coaxed out of the tube, squeeze or scrape it onto a palette. It will dry slowly on the palette, but remain usable like a watercolor pan. Watercolor paint remains watersoluble when dry, so you can always "reactivate" it with a wet brush. (Unlike acrylics!)
If you're determined to get the paint into a tube-like consistency again, grind up the hardened paint with a Muller, and mix it up with some gum arabic. It may never be as smooth as the original, but a granular or gritty paint can be useful for textures, for instance sand or rust.