T-SQL Update Random String Value in Column
One possible approach is to reverse the full filenames and find the actual file names.

Table:

CREATE TABLE Data ([FileName] varchar(100))
INSERT INTO Data([FileName])
VALUES
    ('C:\ABC\Files\AA.dat'),
    ('C:\ABC\Files\AA.Unbound.qp'),
    ('C:\DEF\AA\GGG Build 1 Modules\Random123.qpp'),
    ('C:\DEF\AA\GGG Build 1 Modules\Random456.qpp'),
    ('C:\GH\DC.Random789.qpp'),
    ('C:\IJK\Random987.qpp'),
    ('Random000.qpp')


Statement:

UPDATE Data
SET [FileName] = CONCAT(
   'D:\FILES\',
   REVERSE(LEFT(REVERSE('\' + [FileName]), CHARINDEX('\', REVERSE('\' + [FileName])) - 1))
)


Result:

FileName
D:\FILES\AA.dat
D:\FILES\AA.Unbound.qp
D:\FILES\Random123.qpp
D:\FILES\Random456.qpp
D:\FILES\DC.Random789.qpp
D:\FILES\Random987.qpp
D:\FILES\Random000.qpp