"Where" do you want to change the logo?
The one in the top-left corner of all the application screens is just in the UI themes CSS (#ppm_header_logo) that you edit in the administration menu (no "files" under "webroot" needed).
( some discussion on this here ; Including Company logo in Clarity Header section v13.1 )
Good idea for a Tips !
dorol01: Good idea for a Tips !
Tip : read the "Studio Developer's Guide" ; in section UI Themes › Custom UI Themes › Custom UI Theme: Main Page Header
as dave suggests in the linked post, you can easily change the logo.
However If you want more than that you will have to wait fir the upcoming 13.3 release, more customisation is availible to login screens etc
am1: ...more customisation is availible to login screens etc
and that would be detailed in the UI Themes › Custom UI Themes › Custom UI Theme: Login Page part of the the "Studio Developer's Guide" (v13.3 version, as released yesterday!)
link to the 13.3 bookshelf here (hope that saves the searching!)
https://support.ca.com/cadocs/7/CA%20Clarity%20PPM%2013%203%2000%20%20On%20Premise%20-%20Restricted%20Content%20-ENU/Bookshelf.html
There is a little on modfifying the login page at
https://support.ca.com/cadocs/0/CA%20Clarity%20PPM%2013%203%2000%20%20On%20Premise%20-ENU/Bookshelf_Files/HTML/chgimpact/index.htm?toc.htm?2232623.html
Martti K.