leafs are going yellow?

It sounds as though it is just taking on it's Autumn tints and soon it will loose it's leaves until next spring. They are not evergreen bonsia or no bonsia.

There are two varieties of Ulmus parvifolia.The larger leaved is hardy, but the smaller 'Yatsubusa' with leaves approx 1cm. long is evergreen , however even evergreens need to loose their older leaves. One cause is if it has had a move from warm to cool or vice versa, this can cause leaf fall. Is it well drained,? Sometimes overwatering can cause damage, as can excess ive fertilizer . Check to see if stem is green beneath the bark if it is then it will recover, or you could repot if no drainage holes in the container.
If it is the small leaved variety it needs frost protection
Good luck..