Log in to your reseller partner portal, here. On your "My Account" (ref. 1.) tab, click on "My Details" (ref. 2.).
On the right side, you will have 4 fields where you can upload different-sized logos.
Be sure to pay attention to the format and the dimension requirements.
A quick search will help you find answers, to most of the FAQ's.
If you are unable to a find solution from the knowledge base centre, please contact
your service provider for technical assistance.