declare @value nvarchar(50) = N'100'

declare @xml xml = N'
<Parameters>
	<Parameter Name="ID">
		<Values>
			<Value>123</Value>
		</Values>
	</Parameter>
	<Parameter Name="ID2">
		<Values>
			<Value>1</Value>
			<Value>2</Value>
			<Value>3</Value>
		</Values>
	</Parameter>
	<Parameter Name="ID3">
		<Values>
			<Value>333</Value>
		</Values>
	</Parameter>
</Parameters>'

Select @xml

declare @sql nvarchar(1000)
Set @sql = N'Set @xml.modify(''replace value of (/Parameters/Parameter[@Name=ID3]/Values/Value/text())[1] with "'+ @value +'"'')'

EXEC sp_executesql @sql, N'@xml xml OUTPUT', 
				   @xml = @xml output
				   
Select @xml