Update YUMI on the flash drive, add and remove ISOs

You'd have to ask the publisher. I just packaged it in our format so our users can easily keep it up to date. I think there are some docs in the app and on the publisher site. I don't use YUMI myself.

Dunno here.

Version never changes. It was 2.0.1.8 on 2015-06-17 and was again on 2015-10-24.

1. How do I add, remove or update ISOs (add never versions of the same distro) on the USB flash drive?
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This is basic YUMI usage.

To add a Linux distro:
- start YUMI
- in the Step 1 box, select your thumb drive
- in the Step 2 box, select the desired distro
- in the Step 3 box, browse to and select the matching ISO
- enable persistence if you want (I haven't had good luck with persistence so I usually leave it at zero)
- click the Create button

To remove a Linux distro:
- start YUMI
- click the 'View or Remove Installed Distros' check box to enable Uninstaller mode
- in the Step 1 box, select your thumb drive
- in the Step 2 box, select the distro to remove
- click the Remove button

To update a distro, just use YUMI to remove the old version of the distro and add the newer version. If you have persistence enabled you will loose it, but if persistence is enabled (and working for you) and you've been keeping up with Linux updates you don't really need to update; the distro is already up-to-date.
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2. How do I update YUMI itself on the same USB flash drive already populated with ISOs?
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YUMI is a simple executable. To update it just copy the new executable to the thumb drive.

That said, you don't need YUMI on the thumb drive at all to boot linux. Once you've set up your distros you can discard YUMI if you want and just download it again the next time you need it. Or you can keep it around - it won't make any difference.