How to make 0 in all column in a specific table where value is null using MSSQL

Use ISNULL if you want to see the NULL as 0. Like this

SELECT ISNULL(Column1,0) FROM YourTable

or what you need is to update the value as 0 if NULL and keep the value as it is otherwise. these use a case in the update statement. Like this

Update YourTable
SET Column1 = CASE WHEN Column1 IS NULL THEN 0 ELSE Column1 END,
Column2 = CASE WHEN Column2 IS NULL THEN 0 ELSE Column2 END

and so on for the rest of the columns. Or this is also possible

Update YourTable
    SET Column1 = ISNULL(Column1,0),
cOLUMN2 = ISNULL(Column2,0)

Update table Set column1 = coalesce (column1,0), ....

You can use something like this. It is elegant, but it will update all columns in the table. Huge tables might kill the server

DECLARE @TableName sysname = 'tablename'
Declare @UptQuery varchar(max)

Select @UptQuery = stuff(T.X.query('name').value('.', 'varchar(max)'), 1, 1, '')
from 
(Select ','+name + '=ISNULL('+name+', 0)' name from
    sys.columns where object_id = object_id(@TableName) for xml path(''), type) T(X)

exec ('Update ' + @TableName + ' set ' + @UptQuery)

Old answer It will go through all columns for a table and update everything with 0 if it is null. It is a lot of updates, and I think it is still better to design the table correctly from the start.

DECLARE @TableName sysname = 'tablename'
Declare @ColName sysname

Select name into #temp from sys.columns where object_id = object_id(@TableName)
while(0 < (Select count(1) from  #temp))
BEGIN
  SET ROWCOUNT 1
  Select @ColName = name from #temp
  SET ROWCOUNT 0
  exec('Update ' + @TableName + ' set ' + @ColName + ' = ISNULL('+@ColName+', 0) where ' + @ColName + ' is null')
  delete #temp where name = @ColName
END

   declare @tableName varchar(30)
    set @tableName='MyTable'

    DECLARE @MakeString AS NVARCHAR(MAX)
    SELECT @MakeString= 
    (SELECT cname + ',' AS 'data()' 
    FROM ( select  COLUMN_NAME +'= isnull(['+COLUMN_NAME+'],0)' as cname from INFORMATION_SCHEMA.COLUMNS 
    WHERE TABLE_NAME = @tableName
                )  as ccc
    FOR XML PATH('')) 

    SET @MakeString = LEFT(@MakeString, LEN(@MakeString) - 1)

    DECLARE @Sql AS NVARCHAR(MAX)
    set @Sql='Update '+@tableName+'
    SET '+@MakeString+''

    EXEC(@Sql);

UPDATE Table SET ColumnName1=0 WHERE ColumnName1 IS NULL

...