Hello Elimelech,
Thanks for contacting us. Can you check if these folders are re-writable:
/administrator/components
/administrator/language/en-GB
/administrator/modules
/cache
/components
/language/en-GB
/media
/modules
/plugins
/plugins/system
/templates
If its still not working, please provide us an FTP login and temporary admin login so that we can manually install. Send it please to our email
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If you want to upgrade to a new version, there's no need to remove the existing MJ. Just install the new version.
Best,
John Mark C.
Mobile Joomla! Support