Id   Code
1    TXT.yut.get
2    yep.no
3    gT.YU.iop

Id   Code
1    txt
2    yep
3    gt

Update table_A
SET Code=lower(SUBSTRING(Code, 1, CHARINDEX('.',Code) - 1))

UPDATE table_A
     SET Code = LOWER(SUBSTRING(Code, 1, CHARINDEX('.',Code) - 1))
WHERE Code like '%.%';