Hi Clara probably you are new in prestashop like me
i follow the instructions of the moderator and i fix it.
you have to move the files from your root directory first .
login to your webhost - controlpanel - file manager - yourdomain - and there you will see the folder /prestashop
you have to move all the content of the file out of there in yourdomain file ,
and then if you want delete /prestashop folder
i hope i help you