From cold only.

Solution is here: https://talk.maemo.org/showthread.php?p=1482874 But you need developers mode first and to install the developers mode you need.... :-)

Briefly:
1. enable non-store installations (in menu of N9)
2. install https://openrepos.net/content/ancelad/n9-repomirror (replaces dead links to Nokia sw with live ones)
3. switch on developer mode including automatical install of several needed components (in menu of N9)
4. install https://openrepos.net/content/basil/warehouse (= free app store with up to date sw for N9)