To customize the logo you must go in Nextcloud > Administration settings > Theming > Logo > Upload.
It’ll update the logo for the whole Nextcloud instance including applications like Collabora, etc.
Nextcloud Hub 8 (29.0.0)
COOLWSD version: 24.04.3.1 (git hash: 594b605 (E))
LOKit version: Collabora Office 24.04.3.1 (git hash: 31b6e2c)
Served by: Debian GNU/Linux 12 (bookworm)