Very easy, just add a repository and run apt-get install postresql-9.0 
Dctr Watson explains how:
Installing PostgreSQL 9.0 on Ubuntu 10.04
How to install PostgreSQL 9 on Ubuntu Linux 10.04/10.10
by Jason L Froebe on December 18, 2010, one comment
Jason L. Froebe – Tech tips and How Tos for Fellow Techies
Tips & Tricks for Databases (Sybase, Oracle, MySQL, PostgreSQL, SQLite), Windows, Linux, Solaris, Perl, Java, Bash and so much much more
Segun tu facil pero donde esta el reporsitorio para Ubuntu 10.10.