Replace nulls in column with Auto Increment by number

Question

Hi,

 

I have one table with two columns. S.No and Date. S.No has null values. 

S.No	Date(DD-MM-YYYY)
Null	19-10-1993
Null	12-03-1995
Null	15-02-2017
Null	26-08-2010
Null	01-06-2016

 

 I need to replace these nulls with number in the sequence based on date.

 

Can you please help on this?

 

Thanks in advance!!!