If you want to edit the model manually, here is how to edit the model in Unity!
You can download the VRM model on this site, create a project on Unity, and drag & drop an addon called "UNIVRM" + the model file on assets, which you then place in the "hierarchy" (left panel).
Open Prefab (right arrow on the character's name, you can modify some parts of it. The background will look blue), go to root → hips → J_Sec_Hair1_37(or something like that, which will be before the spine), move it a bit until it looks good! You can also open this piece of hair and move every 7 bones it has, you can decrease or increase its size.
After that, find "VRM0" in the top panel and "Export to VRM0.x".