Hi @Roger,
To update the relationship with a value, you need to have that current data in the current screen. One way is to make the button a list > filter the list for the value that you need > max items to 1 > auto refresh on and add the update action into the button to update with the current data.
Check these out : How to have a Form Automatically Set a Relationship | Adalo Resources , Preselecting Dropdown option - #2 by Dilon
Another option is to use a 3rd party tool like MAKE but you need the teams plan in Adalo to update the database through MAKE.
Thank you and have a great day!