You could use EXISTS and a correlated subquery to filter on rows whose UserId has at least one audit event of id 14:

UPDATE h
SET IsActivated = 1
FROM [dbo].HRUser h
WHERE EXISTS (
    SELECT 1 FROM 
    FROM dbo.[UserAudit] a
    WHERE a.UserId = h.UserId AND a.AuditTypeId = 14
)

Note that there is no point reopening the target table in the subquery; you just need to correlate it with the outer query.