update field

Two methods as below, you may choose what suits in your requirement:

create table test (SNNO_NR nvarchar (8),  ID    int)

Insert into test Values
('5720990',268942),
('5720991',268943),
('5720992',268944),
('5720993',268945),
('5720994',268946),
('5720995',268947),
('5720996',268948)

update test Set SNNO_NR = '0' + SNNO_NR 

Select * From Test

--OR

update test Set SNNO_NR = Right('00000000' + SNNO_NR,8) -- 8 is the size of the datatype

Select * From test 

Drop table Test