First, determine the "License Check Date" of the version you want to install. In this case, the version 12.1.2 was released on 2016-03-18 (License Check Date). Then, determine your "Service check date" by executing the following command from CLI :
> grep "Service check date" /config/bigip.license
The output appears similar to the following example:
> Service check date : 20151008
Since the "Service check date" (20151008) is older than the "License Check Date" (2016-03-18), a license a reactivation is needed before upgrading. To reactivate, follow the steps under paragraph "Reactivating the system license" from the link given above.
Step 5 : Use "iHealth Upgrade Advisor" to determine if any configuration modification is needed before/after the upgrade
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Step 6 : Backup the configuration by generating a UCS archive and download it on a safe place
a) If are using the "Configuration Utility", follow the procedure under "Backing up configuration data by using the Configuration utility"
b) If you prefer using CLI, follow the procedure under "Backing up configuration data using the tmsh utility"
Step 7 : From the release note of the version you wish to install read the "Installation checklist"