You should not even be considering updating the system tables unless you are willing to deal with a corrupt and unusable database afterwards. Say you did update the xtype, now the system tables shows it's an int (4 bytes), but the data pages themselves still have 8 bytes (bigint) allocated to the columns. That'd cause one hell of a mess and likely some nasty messages.
If you want to convert the column from bigint to int, you do it the same way as the conversion from int to bigint.
ALTER TABLE <Table Name>
ALTER COLUMN <Column Name> <new data type> <nullability>
As an example
ALTER TABLE MyTable
ALTER COLUMN SomeColumn INT NULL
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