How can I safely remove Windows from a dual-boot setup?

The usual approach is to boot from a Linux live USB that includes GParted, delete the Windows and Windows recovery partitions, then expand the Linux partition into the unallocated space. Don’t resize the mounted system partition from within your running installation, and be absolutely certain which partitions belong to Windows versus Linux or the EFI boot setup.

Whether this is straightforward depends on the partition layout. If the unused Windows space is directly next to the Linux partition, expanding it is usually manageable. If the EFI partition or another partition sits between them, moving partitions becomes riskier and a clean Linux reinstall may be simpler. During a reinstall, back up your data and choose the option to use the entire drive.

You don’t necessarily have to merge the space into the root partition. You could delete or reformat the old Windows area as ext4 and mount it for games, videos, music, or other bulk storage. If you use LVM, you may instead be able to add the freed space to the volume group and extend the logical volume.

Back up anything important first, preferably to another physical drive. A disk image or at least a verified file backup gives you a way back if partition changes damage the filesystem or bootloader. A Timeshift snapshot can also help restore the Linux installation, although it shouldn’t be your only backup.