Re: how upgrade an old bp theme, please?
If you relied a lot on the BP 1.0 or 1.1 theme structure, then you’d need to install the BP Backwards Compatibility plugin:
https://wordpress.org/extend/plugins/buddypress-backwards-compatibility/
I should note I have no experience using this plugin whatsoever.
hnla is right; if your theme isn’t too custom, I would recommend building a new child theme of the bp-default.