Using the trial/eval keys, you can re-use them 3 times for up to 90 days.
After 90 days or before, you will have to backup the config (NOT a flash backup), wr erase all, and then either re-configured (great practice) or paste in the new config.
You cannot use the flash backup as it will restore the old key timers and will show expired upon reboot.
Backup Config Only
In the WebUI
1.Navigate to the Maintenance > File > Copy Files page.
2.Select the source where the file or image exists.
3.Select the destination to where the file or image is to be copied.
In the CLI
copy startup-config flash: <filename>
copy startup-config tftp: <tftphost> <filename>
copy running-config flash: <filename>
copy running-config ftp: <ftphost> <user> <password> <filename> [<remote-dir>]
copy running-config startup-config
copy running-config tftp: <tftphost> <filename>
The default.cfg cannot be overwritten at runtime through tftp/ftp.
Copy the file into the controller under a different name, then set boot boot config-file to that file.
After a reboot, if you would like this to be your default.cfg, set the boot config-file back to default.cfg
then issue a write mem.