You can try two things.
bpy.data.collections['YOUR_COLLECTION_NAME']
or
bpy.context.scene.collection.children[0] #or the index of your collection (that is the first child collection of the master scene)
I would mess around in the python console with autocomplete to figure out the way to do what I wanted to do.
eg – to change the name of your collection… (D is a shortcut for bpy.data in the console)
D.collections['Collection'].name = 'Test'
Good luck