You could install Ubuntu-MATE over your pre-existing Ubuntu installation if you insisted on formatting prior to installing MATE. I would suggest installing Ubuntu-Mate 18.04.3 whose iso file can be found at
http://cdimage.ubuntu.com/ubuntu-mate/releases/18.04.3/release/
A very helpful guide to installing MATE can be found at
WARNING 1:
BACKUP ALL YOUR DATA BEFORE YOU START TO INSTALL IN CASE THINGS GO WRONG!. SAVE ALL YOUR IMPORTANT FILES AND FOLDERS TO EITHER AN EXTERNAL USB DRIVE AND/OR TO A 2ND HARD DRIVE ON YOUR PC. DO NOT SAVE ANYTHING ON THE SAME DRIVE YOU INTEND TO USE FOR LINUX UNLESS YOU UNDERSTAND/KNOW WHAT YOU ARE DOING!. ONCE YOU START PARTITIONING AND YOU COMMIT TO FORMATTING YOUR PARTITIONS, THERE IS NO GOING BACK!.
WARNING 2:
IF YOU HAVE A RECOVERY PARTITION AND HAVE NOT CREATED ANY RECOVERY DISKS FO…
Good luck Internaut.