There are two approaches:
A) Open File → External Data → Pack Resources and save the file. This ensures all external data is packed into the blender file. Then re-upload the blender file and start a rendering.
B) First move all external files into the same folder as your blender file (or a subfolder of). Adjust all paths of the external assets and then open File → External Data → Make Paths Relative and save the file. Then re-upload the project and start a rendering.
Either approach is fine to use. The first approach is probably a bit more convenient while the second approach ensures less data has to be uploaded on every change.
Make sure to save the blender file after updating.