Something like this will have to be done through a script. You must find a common parameter for the computers you wish to update that is not shared by other computers.
An example script can be seen below. This script can be executed in the Lansweeper configuration console under Lansweeper\Database Scripts. It sets the "Branchoffice" value for computers starting with "LAN" to "Office 1".
INSERT INTO tblCompCustom (Computername)
SELECT Computername
FROM tblComputers
WHERE (Computername NOT IN (SELECT Computername FROM tblCompCustom AS tblCompCustom_1))
GO
UPDATE tblCompCustom
SET Branchoffice = 'Office 1'
FROM tblCompCustom INNER JOIN
tblComputers ON tblCompCustom.Computername = tblComputers.Computername
WHERE (tblComputers.Computer LIKE 'LAN%')
Please note as well that you should practice caution when making database changes.
Always back up your database before applying any changes.