Insert sequential number in MySQL

As I said in comments you can update every row with its row number,

Here is a link to how to calculate rownum in MySQL.

To rephrase:

update player,
       (select @rownum:=@rownum+1 ‘rank’, p.* 
        from player p,
        (SELECT @rownum:=0) r 
        order by score desc) player1
 set thatColumn= rank
 where player.id = player1.id

Run the following queries to have incremented value in yourField column:

SELECT @i:=0;
UPDATE yourTable SET yourField = @i:=@i+1;