Of course, you can adjust them, sometimes to avoid unfragmented packets discarded, reasonable adjustment of MTU/MSS is necessary.
The command for adjusting the MTU value of an interface or a tunnel is just mtu.
please refer to https://support.huawei.com/hedex/hdx.do?docid=EDOC1100075578&id=mtu&text=null&lang=en.
And the command for adjusting the MSS value is tcp adjust-mss.
https://support.huawei.com/hedex/hdx.do?docid=EDOC1100075578&id=tcp_adjust-mss&text=null&lang=en
You can also use the tcp max-mss command to set the maximum MSS value for a TCP connection.
https://support.huawei.com/hedex/hdx.do?docid=EDOC1100075578&id=tcp_max-mss&text=tcp max-mss&lang=en