From your code it looks like you have two field in the item table
“No.” & “Item No.” and you want to replace the currect “No.” with the value in the “Item No.” field.
When changing a field that is the primary key you need to use RENAME
Plus your first post is unclear - are you wanting to change the “No.” field for blocked items only?
What do you want this and what are you ultimately trying to achieve?
And from your code it’s pretty clear you better be doing this in some test database, because you don’t know what you’re doing.