Yes, in most cases you can update the banners and offers on your white label travel portal yourself from the admin area, without waiting on anyone else to do it for you.
The parts that are usually in your own hands are the homepage banner images, the promotional text that sits with them, featured destinations or deals you want to push, and the simple content pages such as about us, contact and your terms. If a blog or offers section is included in your setup, you can normally add and edit those posts yourself too. The practical tip here is to keep your new banner images the same shape and size as the ones already in place, because an image that is much taller or wider than the slot it goes into can make the page look stretched or cut off on a phone.
Where you will need help is anything structural. Moving a section to a different place on the page, changing how the search box looks, adding a completely new type of page, or altering the layout itself is not usually something you can do from the admin area, and how much self editing you get can also depend on the plan you are on. It is also worth knowing that the search results, hotel descriptions and prices come from the suppliers, so those are not yours to edit, only the surrounding content is.
The best thing to do next is ask for a short walkthrough of your admin area while your portal is being set up, so you can see exactly which areas you can change yourself, and keep your original logo and banner files somewhere safe so you always have them to hand.