Your home currency is the one your personal totals, budgets and reports are shown in.
You can change it in Settings → App settings → Home currency, but only until you log your first personal record. After that it's locked, so your history always adds up in one currency, and old totals don't suddenly change meaning.
You can still log expenses in any currency, at any time. NEXO converts them to your home currency for you (see Can I log an expense in a different currency?).
A shared space's currency is separate, and set when the space is created.