Currently I'm running BIG IP LTM 10.2.1 and wanted to upgrade 11.5.3

Hi Yugraj As per the release notes this will be a direct upgrade, We cannot have a direct upgrade from versions earlier than 10.1.0.As per my personal experience i notice some of the configuration/irule syntax has changed and the configuration may not load. You can use the load sys config verify command to troubleshoot this issue.Make sure you have the valid service contract available before starting the upgrade.If this is in HA mode first do the activity in standby device and troubleshoot in case of any issues.Keep in mind that HA configurations format is changed in version 11and please prepare for the same before proceeding with upgrade. Please find the below release notes section which says its a direct upgrade. Upgrading from version 10.1.0 (or later) or 11.x When you upgrade from version 10.1.0 (or later) or 11.x software, you use the Software Management screens in the Configuration utility to complete these steps. To open the Software Management screens, in the navigation pane of the Configuration utility, expand System, and click Software Management. For information about using the Software Management screens, see the online help.