MySQL Update by Row number

MySQL Update by Row number

I need to update rows by their number(not AI ID, cause some of the rows
may will be removed). How can I do this? I mean something like this:
UPDATE cars SET idx = value WHERE row_number = i
I would do this in a 'for' statement, and i is the integer of my
statement. So I would update every row in the statement.
Sorry for my bad english, and thanks!