设置中安装应用未知权限在“系统安全”设置项中，具体设置步骤如下：

1、打开MIUI系统设置应用界面，点击“更多设置”选项；

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2、在打开的设置页面中，选择“系统安全”设置项；

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3、在系统安全设置页面中，点击“特殊应用权限”设置；

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4、在特殊应用权限页面中，点击“安装未知应用”选项；

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5、在安装未知应用页面列表中，选择要进行设置的应用；

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6、最后，在打开的页面中，点击开启对应的按钮开关，即可允许安装来自此来源的应用。

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