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

The layout of the drive matters. If the unallocated space ends up directly beside the Linux partition, expanding it should be fairly straightforward. If the EFI or other required partitions are between them, you may need to move partitions, which is riskier. A full disk image or reliable backup gives you a way back if something goes wrong.

You can usually do this with GParted, but first identify exactly which partitions belong to Windows and which are used by Linux or the EFI boot system. Boot from a live USB so the Linux partitions aren’t mounted, delete the Windows partitions, then expand the adjacent Linux partition into the unallocated space. Make a complete backup first, since moving or resizing partitions can result in data loss or a broken bootloader.

Since the installation is only about a year old, backing up your files and reinstalling Ubuntu using the entire drive may be the cleanest option. It removes the old Windows partitions and boot entries without requiring complicated partition moves. If you use LVM, another option is to add the freed space to the volume group and extend the logical volume, followed by resizing the filesystem.

Instead of resizing the Linux installation, you could delete the Windows partitions, format the freed space as ext4, and mount it somewhere such as your home directory for games, videos, or other large files. This avoids moving the existing Linux partition and is generally less risky, though it won’t make the root filesystem itself larger.