How to repair gems installed with 2.3 after ruby update to 2.5

I’ve updated ubuntu 16.04 to 18.05.
In that process ruby was updated from 2.3 to 2.5.
It seems that’s breaking the gems.

The gems are not running, while they are linked to the old ruby:
bash: /usr/local/bin/jekyll: /usr/bin/ruby2.3: Defective interpreter: File or directory not found

But I’m also not able to uninstall the gems. They are searched in the new gems dir, for examle:
gem uninstall jekyll
gem 'jekyll' is not installed

How can I fix the old gems to work with the new ruby?