Follow these steps to customize permissions for a specific subsite in SharePoint:

1. Go to the root site of the site collection.
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2. Click the Settings gear on the top ribbon or
   use the Site Contents link to expand/collapse the quick launch.
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3. Click Site Settings.
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4. Under the Users and Permissions section of the Site Settings
   page, click Advanced Permissions.
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5. On the ribbon of the Advanced Permission page,
   click on Permissions.
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6. In the tree view, select the subsite for which you wish to
   customize the permissions.
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7. Click the Edit User Permissions button.
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8. Check or uncheck the permissions as needed.
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9. Click Stop Inheriting Permissions to save your changes.