Please try as mentioned here:
https://bitcoin.org/en/release/v0.18.1#wallet-gui Unless you're using ancient Bitcoin Qt (0.7.x or lower), the upgrade process will be done automatically and all you have to do is wait.
If you encounter any problem on automatic upgrade, please share debug.log
It is also said in the link provided that "Upgrading directly from 0.7.x and earlier without redownloading the blockchain is not supported." So yeah, that's one kind of an extra step OP needs to take before continuing with the upgrade.