The Data Management – File Locking page of the Preferences dialog includes controls related to file locking.
The Data Management – File Locking page is part of the main Preferences dialog that is accessed by clicking the control in the upper-right corner of the workspace then selecting the File Locking entry under the Data Management folder.
You will receive a warning when opening or saving files that are owned by another instance of Altium Designer. Use the following options to select the type of warning message you want to receive in each case. Select Warning in Dialog box to receive a warning dialog; select Warning in Message Panel to receive the warning message in the Messages panel.