@DanialN
You can only change the home currency before registering any transactions. You should open a new blank QBO account, enable the multi currency, and change your home currency to SGD if required.
https://quickbooks.grsm.io/UK
Then you may have 2 ways migrating data from your old account before cancelling it.
1. Utilize a migration tool.
https://transactionpro.grsm.io/qbo
2, Purchase a 3rd party migration service.
