A: The user ID is the value for updating this field (not the name).
If the field has Users can select multiple users turned on, put all the user IDs in one cell, separated by commas (for example 12,34).
A cell that contains anything other than user IDs, such as a user's name, is not imported, and any value the field already has on that record is removed. The import does not report this as an error.
The ID is not checked against your list of users, so make sure each ID belongs to the right user in your account.
An export from Upsales lists user names in this column. Replace the names with user IDs before you import the file again.