1. Trying to update 2.5.26 to 2.6 with this instruction https://code.google.com/p/mollify/wiki/Installation
2. Opening http://<fqdn>/fm/backend/update/ and see nothing

I turned on PHP error reporting. Now i see "Parse error: syntax error, unexpected '[' in /var/www/html/fm/backend/include/filesystem/FilesystemController.class.php on line 93" on update page

I've released 2.6.1 that has installer fixes.

The error you had, I still cannot see that happening, but made a "blind" fix. It is possible, I guess, that some PHP version cannot handle shortcut notation for function call and index accessor in the same line, like this

$value = $this->getSomeArrayValue()["index"];

so I separated them into two lines

$array = $this->getSomeArrayValue(); $value = $array["index"];

This should not be required, but hopefully this is enough.