How do I change the currency for Chart of Accounts in Deskera Books?

You cannot change the currency for Chart of Accounts, as all financial reports that are generated using these Chart of Accounts or Ledgers have to be in the base currency. This is the reason all chart of accounts are created only in the base currency

How do I pass transactions in a foreign currency?

Even though changing the currency for Ledgers/Chart of Accounts is not allowed, you can still create transactions in a different currency and specify the exchange rate. The following transactions for example - Journal Entry, Direct Expense, Advance Payment, Pre payment, etc can also be made using the existing ledgers but in a different currency