Regrettably, I followed a tutorial and set PYTHONPATH to qgis\apps\python. After doing so, typing python -V in the OSGeo4W Shell gives Python 2.7.14.
Previously, the result was Python 3.7.0, which is the version I want.
I deleted the PYTHONPATH variable and rebooted my pc, but version 2.7 remains the default in the shell.
How can I restore Python 3 to the default again.