Help on SQL Query

Typically this type of setup you'd configure with an automatic ID generation on insert (ie, you insert the product name into the table and then the database automatically assigns the ID according to the rules you define.)
For the actual query, this is a solution, but it is very dependent on the data in question:
select ProductName,
case when ProductID is null then
row_number() over (order by productid) * 10
else ProductID
end as ProductID
from products
This assumes that all IDs with null values are at the end of the script (and obviously that they're just separated by 10 between them).
For a more robust solution, I would probably look at nested queries / CTEs and / or using a stored procedure assuming this has to be done entirely in SQL. If you are trying to update a table, I would likely use something like Python to handle the logic of the values and continue from there.