Open Word and select Account (or File > Account)
Under Product Information, click the About Word button.

For detailed instructions, see this Microsoft article.

Open a Word document.
Click the Insert tab and click Add-ins and then select Get Add-ins.
On the STORE tab, type Nepali in the search box and click the search icon. Nepali Spell Checker for Microsoft Word shows up in the result after the search completes.
Click the Add button.

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Common errors and resolutions

Word 2019: When installing the add-in, I see "Error loading Add-ins" in the status bar.
File > Options > Trust Center > Trust Center Settings > Privacy Options
Make sure the "Turn on optional connected experiences." option is checked.
Click OK and restart Word, and install the add-in again.

Word 2021: I do not see the STORE tab when I click Get Add-ins.
File > Options > Trust Center > Trust Center Settings > Privacy Options > Privacy Settings
Make sure the "Turn on optional connected experiences." option is checked.
Click OK and restart Word, and install the add-in again.