Replace specific part of a string
Given your test data, this will parse out the section of the string you want between either the first semi-colon after the PATH=, or the end of the string if that doesn't exist. From here, you should be able to use an update to replace that section of the string.
Further reading: Get The Text Between Two Delimiters
SELECT 
    x.*,
    SUBSTRING(x.PARAMETERLIST,
        CHARINDEX('PATH=', x.PARAMETERLIST) + LEN('PATH='),
        CASE WHEN x.PARAMETERLIST NOT LIKE '%PATH=%;%'
             THEN LEN(x.PARAMETERLIST)
             ELSE CHARINDEX(';', x.PARAMETERLIST, CHARINDEX('PATH=', x.PARAMETERLIST) + LEN('PATH='))
                  - LEN('PATH=') - CHARINDEX('PATH=', x.PARAMETERLIST)
        END
             ) AS parsed_string
FROM
(
    VALUES
        ('KF=138;SCANANDSEND=TRUE;ENDBARCODE=999999999999;PATH=K:\SSTEST;BARCODELENGTH=8'),
        ('KF=138; PATH=s:\'),
        ('DESIGNATION=JOB; KF=81; PATH=w:; SENDALLDATA=TRUE'),
        ('KF=76; PATH=\srv05\L3OERAPMFC; SENDALLDATA=TRUE'),
        ('KF=386; PATH=W:; SENDALLDATA=TRUE')
) AS x(PARAMETERLIST);