Change registration day only

Considering that DATAINCLUSAO is of type datetime you can try the following UPDATE :

UPDATE tabela SET datainclusao = CONVERT(
    datetime, FORMAT(datainclusao, 'yyyy-MM-01 H:mm:ss')
);

In the FORMAT function I am formatting the date into a format that datetime will understand by changing the day part.

Then with CONVERT I convert the date back to datetime because the return of FORMAT is nvarchar .

You can use the DATEFROMPARTS , to change only the day:

UPDATE tabela 
  SET datainclusao = datefromparts(year(datainclusao), month(datainclusao), 1)

Here, I took the year itself and the month of the date and only set the day as "1", since the function accepts (year, month, day) as parameters.

You can update the data by update yes.

You have to turn the date into text, adjust the day, and turn the value back to date.

I know this way, if I have others, I will learn.

Test with SELECT

select 
  '01' + substring(convert(varchar(25), GETDATE(), 103), 3, 8),
  convert(varchar(12), GETDATE(), 114),
  convert(datetime, '01' + substring(convert(varchar(12), GETDATE(), 103), 3, 8), 103) + convert(datetime, convert(varchar(12), GETDATE(), 114), 114)

No UPDATE

update TABELA set DATAINCLUSAO = convert(datetime, '01' + substring(convert(varchar(12), GETDATE(), 103), 3, 8), 103) + convert(datetime, convert(varchar(12), GETDATE(), 114), 114)

Complementing responses:

From SQL Server 2012 , there is the function: DATETIMEFROMPARTS

Syntax:

DATETIMEFROMPARTS ( year, month, day, hour, minute, seconds, milliseconds )

Example:

UPDATE tabela 
  SET DATAINCLUSAO = DATETIMEFROMPARTS(YEAR(DATAINCLUSAO), MONTH(DATAINCLUSAO), 1, DATEPART(HOUR, DATAINCLUSAO), DATEPART(MINUTE, DATAINCLUSAO), DATEPART(SECOND, DATAINCLUSAO), DATEPART(MILLISECOND, DATAINCLUSAO))

PS: could only use DATEPART to fetch all parts .

Other functions used:

DATEPART