Hi Angela,
go to Design Studio, look for your landing page templates, open them (Edit) and anywhere in the code is:
<img src=" and then the link for your old logo. (If uploaded in Marketo you can also just look for the URL and search for it in the template). Change the link to the new upload and your logo ist changed.
Best regards
Steffen
Erica's method is by far the easiest as it will update on your landing pages, emails, etc... anywhere that image is used. Just make sure that you use the same file name, and select "Overwrite existing files" when you go to upload the image.