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.
You could also go to the logo's location in design studio and replace it with the new version. It will keep the same url and name, but the logo will be replaced everywhere it's used.
If you find and select the file you can use the button Replace Image or File and in the popup choose the new file. Then you don't need to care about the file name :)