If you want VPOP3 to store the database backups somewhere else, for instance, to stop filling up the installation drive, you can change this simply. Go to the VPOP3 settings, then Settings → Database → Backups.
The Backup Target File option tells VPOP3 where to store the backups, and what to call them. The default is DBBack-%w.dmp which tells it to store them in the VPOP3 directory, with the specified filename with the '%w' replaced by the week-day number.
If you want VPOP3 to store the backups with the same name, but in the d:\vpop3backups folder, you could set the Dump Command Target File to d:\vpop3backups\DBBack-%w.dmp
If you want the backups stored on a monthly rotation, you could use d:\vpop3backups\dbback-%d.dmp (the '%d' is replaced by the day-of-month), etc
Storing backups on a USB drive
You can easily specify an external (eg USB) hard drive using the above method, simply specify the USB hard drive location as the target file.
Storing backups across the network
If you want VPOP3 to store the backups across the network (eg on a NAS device), it is a bit more complicated.
Because VPOP3 is running as a service, it does not have access to mapped drives, so you cannot map the NAS to drive Z:, and tell VPOP3 to store the backups as z:\dbback-%w.dmp.
Instead of using mapped drives, you have to use UNC paths, eg \\server\share\path
Also, because VPOP3 is running as a service, it needs to have permission to access the shared folder. You can set the Target File Network Username and Target File Network Password options to the details of an account which can write files to the specified path. Note that just because the current Windows user can write to the file share does not mean that VPOP3 can, because VPOP3 runs as a system service, not as a normal user application.