Using one of the search methods described in Searching for Object Profiles on the MIDM, display the object profile containing the field you want to unlock on the Record Details page.
At the bottom of the page, click Edit EUID.
Scroll to the SBR field you want to unlock.
Click the unlocked padlock icon to the left of the field.
A closed padlock indicates an unlocked field that can be locked. An open padlock indicates a locked field that can be unlocked.
On both dialog boxes that appear, click OK.
Click Save at the bottom of the page, and then click OK on the confirmation dialog box.
The field is now unlocked and can be edited by updates to source records. The link icon appears next to the field again because the field can now be linked to a source record field.