Update a value based on another table's column

[tecpme_casm]

I’m trying to update the ID number for a person based on the ID entered on another table

I have a table named attendance with the data: [key, [person], [meal1], [meal2], [meal3], [documentid])
I have another table called people with the data [key], [name], [documentid]

I have a database of 5,000 people but for a 30% of them the [documentid] for me is unknown (null) I want to update the value in people.[key].[documentid] based on the attendance table

Currently in the table attendance, when the documentid retreived from people[key].[documentid] is empty, displays the column attendance[documentid] but the column in people[key].[documentid] is not updated (I don’t know how to) so the documentid is left over only in the table attendance[documentid]