Remove /dev/sda2 partition and create a new bigger one:
# fdisk /dev/sda
Welcome to fdisk (util-linux 2.23.2).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Command (m for help): d
Partition number (1,2, default 2): 2
Partition 2 is deleted
Command (m for help): n
Partition type:
p primary (1 primary, 0 extended, 3 free)
e extended
Select (default p): p
Partition number (2-4, default 2): 2
First sector (1026048-3906917807, default 1026048):
Using default value 1026048
Last sector, +sectors or +size{K,M,G} (1026048-3906917807, default 3906917807): +512G
Partition 2 of type Linux and of size 512 GiB is set
Command (m for help): p
Disk /dev/sda: 2000.3 GB, 2000341917696 bytes, 3906917808 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 262144 bytes / 524288 bytes
Disk label type: dos
Disk identifier: 0x000b9cd2
Device Boot Start End Blocks Id System
/dev/sda1 * 2048 1026047 512000 83 Linux
/dev/sda2 1026048 1074767871 536870912 83 Linux
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
WARNING: Re-reading the partition table failed with error 16: Device or resource busy.
The kernel still uses the old table. The new table will be used at
the next reboot or after you run partprobe(8) or kpartx(8)
Syncing disks.