>>292416049
In blender, delete the parts of the leg mesh you don't want, but don't delete the bones off the armature. Then take your new boots, add the armature deform modifier "with empty weights", go into edit mode, select all the verts in each boot and assign to the vertex group for your left/right leg bone respectively.

Alternatively, you can delete the leg meshes off the model in blender, then in unity (assuming each boot is a separate gameObject), drag the boot under the Armature->Hip->Left/RIght Leg gameobject and change its transform until it looks right.