Replace one Ubuntu with another from USB pen drive
I think there is no easier way than this illustrated here.
In short:
After Downloading Ubuntu.
Open the dash and search for Startup Disk Creator (for the dash click the icon on top right).
- Start the Startup Disk Creator and Click on
Other
Then select
openand search the location that you have downloaded Ubuntu (normally in ~/Downloads)Select the USB stick in the bottom box and click 'Make Startup Disk'.
Taken from this excellent guide.