Change UEFI boot sequence

Dear All:
I use a Inspiron 15-3537 (W8.1). I prepared my USB stick with an AV program (iso). When I open the Boot Manager (F12), I see this menu:
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Bootmode is set to UEFI
Secure boot on
UEFI Boot
Windows Boot Manager
UEFI Onboard LAN IPv4
UEFI Onboard LAN IPv6
EFI USB
Other Options
Enter Setup
Peripheral Device Settings (OPROM)
Change boot mode settings
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When I choose EFI USB, this message appears:
EFI USB1 has been blocked by the current security policy
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How do I go from there in order to let the AV program run from the USB stick?
I appreciate your comments
STAY SAFE
Henry