There are two ways to do this.
The recommended way is to go into the second scene, click the Exporting tab, then check 'Append to Previous Scene'
You can then click Next lower-left to move to the next scene and repeat. (If the chapter title changes lower right and the 'append to previous' is greyed out, you've moved to the first scene in the next chapter). When you export your file, the scenes will save without breaks in between them, as though you wrote them in one slab of text.
The other way, which I don't recommend, is to select multiple scenes in the main screen list, then click the Scene menu, then click Combine Scenes. If you do that you end up with a single scene in the chapter with all the content. However, if you have to come back later and edit something, this ends up no better than having the chapter in a word processor.
Finally, you can just copy/paste the scene content from each scene onto the end of the one above.
But option one is still the preferred way.
In the latest yWriter Beta you can then right-click any chapter and choose export to RTF or DOCX.
Cheers
Simon