You can restore the file to your Lacerte program if the file was backed up using the backup feature in Lacerte. It's not a single file, but multiple database files. In Lacerte, click the Client menu at the top left of the Client Screen, then select Restore. Change the Restore From path at the top to where the data file is currently located, then select the client and click OK.
If the other firm emailed it to you by using the email file feature in Lacerte, then you can just double click the attachment and it will automatically upload into your Lacerte Client Screen.