how change default margins in template

edit

The issue here must be the naming of the "Default" template. Despite its name, it is not used to create new documents by default :) You must explicitly set it to be default by right-clicking on it in template manager, and choosing relevant option.

To be specific: unless some template is set to be default, LibreOffice doesn't use any templates. It creates new documents from its internal defaults, taking user locale into consideration.

I am on Version: 5.1.6.2 Build ID: 1:5.1.6~rc2-0ubuntu1~xenial2 and just did the following: open template manager, select default (on the documents tab), select edit, the default.ott opens. So far the two of us should have done the same. I am not exactly sure what you mean by point 2 in your description. What I do at this point I edit the default page style, accessible either through Styles>Styles and Formatting or through hitting F11. In the Styles Window select Page Styles right click Default and select Modify. On the Page pane edit the margins and close the result. In my version I am not asked if I want to replace the original default. When I open a new writer document after this the changes I have made are reflected. I know that this doesn't help you unless the problem is hidden in point 2 of your description.

It could also be that you are on another LO version than I am. Therefore: another way to do this is to open a document change the default page style according to your needs and save the document as (File>Save As) default.ott in the /home/<user>/.config/libreoffice/<versionnumber>/user/template. The path to the location of the templates varies depending what OS you are on. I am on Linux Mint 18.