The built-in checkout uses the workspace's selling account. The workspace's business identity is shared by its apps; it is different from an individual app's icon.
Seller name
Rename the workspace in Workspace settings. The platform queues a sync of that name to its connected selling account. Check a newly opened checkout afterwards. If it still shows the previous name, ask support to check the sync; it is not a reason to rebuild the app's checkout.
Seller logo
Where Workspace branding is available, upload the workspace logo there. That page is separate from App Settings → Upload your own, which changes one app's icon.
The checkout-logo sync accepts PNG, JPEG, GIF and WebP. An SVG workspace logo is not supported by that sync, even if another surface accepts it. Use a raster version for the seller logo.
Workspace branding is an account-dependent rollout. If you do not see the branding form, ask support to check availability and the logo on the workspace's selling account. Do not assume that changing an app icon changes the checkout logo.
After changing it, open a fresh checkout and verify the name and image before sending buyers there.