sqlite replace values

i have a table with two columns: one named index and the other is called tilenumber and the tilenumber is populated from the index column except that there is _ which needs to be removed from the column.

The index table has values like 2726BA_11 and the tilenumber should result in 2726BA11 but i seem not to know the way forward, to remove the space i have done the following:

select distinct replace (tilenumber,'_', '') from aug2012

and it shows that the result has been achieved but how do i incorporate the select distinct replace in the update statement which is

update set tilenumber="index"