Can anyone remember which (if any) book it listed how to improve your characteristics like Str & Sta? I seem to recall that it would take a season, but I can't remember if it was only a post on here, or an actual book entry.
The only way to do so that is covered in the rules as far as I know are the creo corpus and creo mentem rituals that are straight forward applications of the guidelines in the main book (there are some example spells presented in true lineages as well as gift of reason in the core book).
Otherwise all that comes to mind are various sets of house rules that I've seen over the years.
If you mean improve them by training, if any place it's in city and guild, the book i dont have, since i've never seen it.
It's a creo corpus or mentem effect to improve characteristics, you could also do it by divine means such as the rings of solamon, or infernal. Presumably by fairy too. There may also be a mystry or two that let you do it.