Update multiple rows with different date for each row
Since you have a primary key userid, you can do it with a query like :

update p 
set passDays = '60',
    passChangeDate = dateadd(dd, offset, '2017-01-01 00:00:00')
from [POSUSER] p
join (
    select  userid, 
            row_number() over(order by userid) / 10 as offset
    from [POSUSER]
    ) t on p.userid = t.userid


This query will work the same with any type of primary key (numeric or not) or any set of unique columns. But if there is no way to build a unique identifier of a row, the query won't work and you'll have to make your update another way.