LEAD function with date scenario

Question

I have multiple files, but lets consider 2 files which have filename and start dates columns.

Start_Date	FileName
2022-01-01	product 1
2022-02-02	product 2

please consider both rows as a separate files data.

Now I wanted to generate a dim table which have below requirement. 1st time when I read the file1 I am looking for dim table like below.

Start_date	End_date	file_name
2022-01-01	null	product 1

for 2nd time when I read the file I am looking for dim table like below.

Start_date	End_date	file_name
2022-01-01	2022-02-01	product 1
2022-02-02	null	product 2

basically I want to change the above row null to 2nd file start_date -1

please help

what I am planning, 1st time I am Inserting the data using below query, and 2nd time load also I am using the same query to insert data first then by using Lead function I am planning to update the table

select first_value(Start_date) as EFFECTIVE_START_DATE, 
null as End_date,first_value(Source) as SRCE_FILE_NAME from pqdf_view 

I am able to write the lead function which is working but How can I update the already inserted column using Lead function. using below query I am able to create a new Column but I want to use the already existing column which is already I inserted using above query data.

select *, LEAD(date_sub(EFFECTIVE_START_DATE,1)) OVER(ORDER BY PRODUCT_QUALITY_SK ASC) as EFFECTIVE_END_DATE from edp_silver.dim_product_quality