Old Grav installation crashes on startup, how to update?

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2

First of all, backup your grav6 folder.
Download Grav and upload the contents to that grav6 without the user folder (user folder contains all your themes, plugins and settings, so try keeping the original contents)
Most likely your old theme will also not work, because of the changes in newer Grav version. If that will be the case, you can try the second option from this post:

Or you can add the following settings to system.yaml to disable auto-escaping:

twig:
  autoescape: false
strict_mode:
  twig_compat: true