To change the default logo go the Thrive Dashboard >> Theme Options >> General Settings and in there you will see the upload button where you can add your own logo.
There are 2 logo upload fields for image logos: one for light and one for dark backgrounds. The logo for dark backgrounds is used on images with a transparent header area, the one for light backgrounds is used for pages that don’t have a title section image and for the floating header.
To get the urls for your logos first upload first your images via the Media library and then click on your image. You will see the url which you can copy and past.