I believe your bonsai is a chinese elm. People who are more expert on bonsai can check me on this....
To really answer about leaves turning yellow, we would need to know more, like how long you have had this tree, where you are and what the climate is, does the tree stay indoors all the time, is the indoors air conditioned, what is the light like where it is, how and how often do you water it, etc.
But I can give you a couple of general suggestions. The commonest reason would be that it is over watered. You may be watering it too often and/or your soil may be too moisture holding. Bonsai soil should be loose, gritty, and free draining. Bonsai should be watered only when they are starting to dry out.
Chinese elm is a semi-evergreen. It does put out new leaves in the spring which means the old ones will yellow and shed. But that should not be going on this time of year (unless of course you are in Australia and it is spring where you are).
Here's a brief article about chinese elm care: https://www.midlandsbonsai.com/chinese-e ... e-35-w.asp