SQL Server - update column values which are null

I have one column in my sql database named as S-No. 
Which has values like following :
 
S-No
____
1
____
2
____
Null
____
Null
____
3
____
Null
____
Null
____
Null
____
Null
____
Null
____
4
____
Null
.
.
 

Now, i want to write query in such a way that, which S-No column contains Null between 1 to 2, update there value to 1..... And which has Null between 2 To 3, update there value to 2... And so on... eg. 3 to 4 -- update as 3, 4 to 5 --- update as 4, 5 to 6...... I hope you will suggest me the query.. i am not able to write..  i hope you guyz will help.

standard sql doesn't have the concept of iterating row by row performing updates on different rows based on the condition of a given row.  You are going to have to write code in your app that iterates through these rows and processes the logic you are after.