To format existing partition by using Partition Manager select existing partition and:
Format partition dialog will appear. In this dialog select proper File System and volume label in necessary. Click Format button to confirm your actions.
Read more: Format Partition
You can rollback these partition changes at any time by using Rollback Partition Changes feature.
Be sure, that all valuable data from partition you want to format are stored in safe location elsewhere - after formatting all data will become unacceptable.