@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.