Use os.chdir
. Here's a full example:
https://colab.research.google.com/notebook#fileId=1CSPBdmY0TxU038aKscL8YJ3ELgCiGGju
Compactly:
!mkdir abc
!echo "file" > abc/123.txt
import os
os.chdir('abc')
# Now the directory 'abc' is the current working directory.
# and will show 123.txt.
!ls