ID    MyText
1     some text; some more text
2     text again; even more text

ID    MyText
1     some text
2     text again

UPDATE MyTable
SET MyText = LEFT(MyText, CHARINDEX(';', MyText) - 1)
WHERE CHARINDEX(';', MyText) > 0

UPDATE MyTable
   SET MyText = SUBSTRING(MyText, 1, CHARINDEX(';', MyText) - 1)
 WHERE CHARINDEX(';', MyText) > 0

SET MyText = LEFT(MyText+';', CHARINDEX(';',MyText+';')-1)

SELECT
    CASE WHEN CHARINDEX(';', MyText) > 0 THEN
    LEFT(MyText, CHARINDEX(';', MyText)-1) ELSE
    MyText END
    FROM MyTable