manjaro-kde-25.0.10-251013-linux612
In BIOS i Have disabled Secure Boot and set boot to my USB stick and seet the boot to my USB stick but it just boots up on the installed operating system. I have verified the download and downloaded the .iso again.
I really don’t think anyone can possibly help you if you don’t bother providing some details.
Have you verified only the download or have you also checked if the data transferred to the stick is correct?
You’ll have to change the boot sequence to let it boot from USB, i.e. interrupt the start when powering the machine up.
Typically it’s F2 or ESC to get into the systems ‘BIOS’ setup or F8 the set the boot sequence.
Refer to your computer’s manual for the exact procedure.
You don’t say how you transferred the ISO to the USB. One recommended way is to use Ventoy for this: [HowTo] Create a Ventoy USB to launch Manjaro (and other) ISO's
For sure, this is to be expected if you don’t stop the boot and select to boot from the USB.
On my system, during boot, I get options to press keys like Esc or F11 to enter setup or boot screens in BIOS and choose what to boot from.
You can also set BIOS to always boot from USB first if one’s inserted.
VENTOY is the GOAT for me.
And in mine – with an ASUS mainboard – the key to press is F8.
This varies according to the mainboard and type of BIOS being used – usually a certain key must be pressed to enter the BIOS boot menu. Consult you mainboard manual or other documentation to discover the correct key to use.
Once you have that, you should be able to select the USB as the boot device, and continue as expected.
If after correctly accessing the BIOS boot menu, and the USB (or it’s boot loader) does not appear, then you may have (somehow) incorrectly created the USB.
In this case, you would need to create a new one – A Ventoy USB as indicated by @Phemisters is suggested – please follow the linked tutorial very carefully.
Please provide your system information as described (below) as soon as you are able to. Until then, your exact mainboard model might help us to point you in the right direction.
Regards.
What follows is from a standard template.
Welcome to the Manjaro community
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Note: By virtue of using the Manjaro forum you acknowledge and agree to follow Rules and Guidelines outlined; so, you really should read them:
Be prepared to provide output from commands when asked. It is equally important to provide as much actionable information as possible in your first post, rather than simply indicating there is a problem.
Simply waiting for others to ask you questions can be counter-productive – typically, nobody has a 
at their disposal – instead, please help others to make informed suggestions based on information you provide.
While information from *-fetch type apps might be fine for someone wishing to buy your computer, for Support purposes it’s better to ask your system directly; 
Output of the inxi command (with appropriate parameters, and formatted according to forum guidelines) will generate information useful for those wishing to help:
a) inxi command (long-form):
inxi --filter --verbosity=8
b) inxi command (short-form):
inxi -zv8
Running inxi within a chroot environment
--color=0 to the long-form command, or…inxi -zv8c0Besides secure boot - it is possible your system firmware - implements a fast boot feature - with such feature enabled your system will skip any usb stick and go directly to the first boot entry in the UEFI boot loader.
Disable the feature - if present - perhaps you need to set the USB stick as the first boot entry too…
Depending on your method of transferring the ISO to usb - it may not be bootable at all - and from your initial topic this seems to be the case.