Hello Paulien
What sort of problems are you experiencing?
I would recommend that you check that all permissions for folders and files are correct.
If you haven't already you should also clear the cache. /core/cache
Although this can be done through the MODX Manager it is often better to do it manually over FTP or SSH.
You can re-upload the 'setup' folder from the installer and re-run it.
You should also check that all Packages are up to date: Extras > Installer > Package Management
These are all fairly general bits of advice though. If you provide more detail about the problems you are experiencing people might be able to provide more focussed, specific advice.
This document covers most of what you need to do for an upgrade:
https://docs.modx.com/revolution/2.x/administering-your-site/upgrading-modx