You can define the default user-directories in the file /home/<username>/.config/user-dirs.dirs.
# This file is written by xdg-user-dirs-update
# If you want to change or add directories, just edit the line you're
# interested in. All local changes will be retained on the next run
# Format is XDG_xxx_DIR="$HOME/yyy", where yyy is a shell-escaped
# homedir-relative path, or XDG_xxx_DIR="/yyy", where /yyy is an
# absolute path. No other format is supported.
#
XDG_DESKTOP_DIR="$HOME/Desktop"
XDG_DOWNLOAD_DIR="$HOME/Downloads"
XDG_TEMPLATES_DIR="$HOME/Templates"
XDG_PUBLICSHARE_DIR="$HOME/Public"
XDG_DOCUMENTS_DIR="$HOME/Documents"
XDG_MUSIC_DIR="$HOME/Music"
XDG_PICTURES_DIR="$HOME/Pictures"
XDG_VIDEOS_DIR="$HOME/Videos"
Just browse to the file in your file manager, open it with a double-click, and change the line you are interested in. Save the file. The changes will take effect after your next login.
Make sure that the destination-folder you specify exists!
You can also use the terminal-command
xdg-user-dirs-update --set DOWNLOAD $HOME/yyy or
xdg-user-dirs-update --set DOWNLOAD /yyy
to achieve this.