How do i changed the security deposit from cash to bank transfer

By stipulating it in the auto message tempalte for new bookings.

Add it as a note to Fine Print also.

Add a photo with text saying prepaid deposit required.

The major benefit of doing this right , is when they fail to prepay in x hours or y days, you can leverage the Request Cancellation feature on details page, and choose option 1.

The guest hasn't paid the deposit/pre-payment. Can I cancel the booking? | Booking.com for Partners 

Reminder Partner Support will never see this.

Posting account or action requests here will result in nothing.

For when its urgent never post here go straight to phone method ,as per below.

 Message or Call BdC PartnerSupport ***

1. In the extranet, go to the ‘Inbox’ tab.
2. Select ‘Booking.com messages’.
3. Right Pane - Click on ‘See Contact Options’. 
4. Choose Account
5. Now choose Other
6. Then at bottom , See all contact options, and click Message or Call
7. As there is no one number for all partners, the number for your nearest BdC Partner Support is then revealed.

*** public main lines via Where you can reach us

Kind Regards