How can I alter default spacing after a period from 1 to 2 spaces

[Jay Freedman]

You can't.

You can set a rule the grammar checker to put a green wavy line under places
where there is only one space, but you'll have to change them manually. To
set this, go to Tools > Options > Spelling & Grammar, click the Settings
button, and set the "Spaces required between sentences" dropdown to 2.

If you're using any proportional font (basically, anything except Courier
New or Lucida Sans Typewriter), just leave it at one space. The font is
designed for that.

--
Regards,
Jay Freedman
Microsoft Word MVP
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[Suzanne S. Barnhill]

There is no way to force Word to put two spaces after a period (or to make
it make you do so), but you can, in the Grammar Settings (Tools | Options |
Spelling & Grammar: Settings...) choose 2 as the number of spaces between
settings. Then, if you use "Check grammar as you type," instances of one
space will be marked as incorrect. FWIW, the majority opinion is that two
spaces are not required between sentences unless you're using a monospaced
font such as Courier New.