It saves as a .wpp file to be able to load itself when you start up WP10 next time - that is only for the WP10 program to use.
*A couple to the right from the site preview icon is "publish to disk folder - this you use if you want to save as html to your hard drive and then use a 3rd party FTP program to upload to the server from there.
*Next to it to the right is the icon "Publish to the web" and that will use WP10's own FTP program.
How is it (please describe) that you are uploading the WPP file?
>> I can't figure out how to change the format from .wbb to html. <<
The wpp file is neither suitable, nor intended, to by uploaded as a web site. The wpp file is WebPlus' "internal" file format, and is totally unuseable as/for a web site.
To "convert" from wpp to a useable HTML/web site file, you must first publish your site by using WebPlus 10's "Publish to Disk" or its "Publish to Web" function.
I strongly recommend that you use the Publish to Web function, as then WebPlus will automatically create the proper web page coding, the proper folder/directory structure on the server, and the proper file naming.
If you try to use WebPlus' Publish to Disk function, then use an FTP client to upload your web site/pages, it is up to you to ensure that you duplicate the required folder/directory sturctutre and that each file is in the correct folder.
Jim B. :> )
When You Earnestly Believe You Can Compensate For A Lack Of Skill By Doubling Your Efforts, There's No End To What You Can't Do.