It is impossible to amend the tip to its parent's state using GUI
Consider following repository:
hg init echo "Original string" > a hg add a hg ci -m "Initial" echo "Changed string" > a hg ci -m "Changed" echo "Original string" > a
Next, in TortoiseHG commit window try to select "Amend" option. There is no files marked as changed. If I unfilter 'clean' files and select file 'a', nothing will be amended also.
On the other hand, using Mercurial command line: hg ci --amend -m "Something" will do as it should.