Modifying partitions
Run this lsblk command to get a clear overview of all your disks and partitions
lsblk -o KNAME,TYPE,FSTYPE,SIZE,LABEL,VENDOR,MODEL
Find the disk you want to modify, and take note of its kname. Note: names like sda, sdb and sdc represent disks. Names ending in a number like sda1, sda2 or sdb1 represent partitions.
Replace sda with the diskname/kname you want to modify. (so not a partition on that disk)
sudo cfdisk /dev/sda
This will open a cli UI where you can select, modify, delete, and create partitions. Also make sure to select the right partition type. Finally write the new config to disk, and quit
Initializing a filestystem
To create a filesystem on a partition you can use mkfs utils, for instance mkfs.ext4 or mkfs.exfat. Obviously, replace the Volume Label name and partition name
sudo mkfs.exfat -n 'Volume Label' /dev/sda1