I tried several times, but I could not change the EDGE start page (to a local HTML file). May be I can change it in the XML? For home button it worked. - I submitted feedback for this already.

Applies to Windows 10

1. Select More () on the address bar, then Settings.

2. Under Open with, select A specific page or pages.

3. Choose one of the default options or select Custom to enter the URL of another page youd like to see each time you open the browser.

4. To save, select the plus button.

• Proposed as answer by Michael ShaoMicrosoft contingent staff, Moderator 22 minutes ago

Applies to Windows 10

1. Select More () on the address bar, then Settings.

2. Under Open with, select A specific page or pages.

3. Choose one of the default options or select Custom to enter the URL of another page youd like to see each time you open the browser.

4. To save, select the plus button.

I figured this out on my own a couple of days ago.  Took forever to get it to "take."  Whoever heard of clicking the plus button to save.  Why not just have a "Save" button? 

User77,

Could you please explain a bit for the local HTML/XML page here?

As dosumi suggested, we could change the Start page under the ... ->Settings:

We could enter a new web address, then click the + to save it, we may also click on the saved page, edit it and then click the save button (save is only available after you edited the address).

Regards