In your document use Replace Ctrl+H.
In the find box type the word Autumn.
In the replace box type the word Autumn.

At the bottom of the dialog click on the More button and then click on the Format button with your insertion point still in the replace box.
Language
Check the box for Do not check spelling or grammar.
OK back to the replace dialog. You should see your new setting underneath the replace box.
Click on Replace All.

That takes care of the current instances.

You can save one of them as AutoText for future ones. Once you are done with this project, remove the AutoText entry. Note that this method risks "infecting" your document with lots of text that will not be checked.