I need to downgrade my account to simple but due to multicurrency i cannot downgrade it and i cannot delete multicurrency as well to downgrade?

You are not allowed to disable the multicurrency and downgrade your plan to QBO Simple Start. You should open a new blank QBO account 

https://quickbooks.grsm.io/EU

then you may have 2 options

1. Utilize a migration tool

https://transactionpro.grsm.io/qbo

2. Purchase a 3rd party migration service.