SHELL

undo git reset HEAD~1

# if you would like to move back the head by one position you could do:
git reset HEAD~

# if you want to move multiple positions at once e.g 3 times back type: 
git reset HEAD@{3}

undo reset HEAD^

# To undo git reset HEAD~

#short answer
git reset 'HEAD@{1}'

#for detail go to this link