How to write update query when subquery returns multiple value?

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SQL-Server-2008R2

I have a query :

update DeviceDefinitionSettingValues set devicedefinitionsettingxrefid=(Select DeviceDefinitionSettingXrefID from DeviceDefinitionSettingXref
where DeviceDefinitionID='6E269410-670F-4E02-9792-E3E15CD9AA49')

The subquery returns mulltiple values.On executing i get this message:
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

What can i do or how can i improvise my query.
I am new to SQL