Need to upgrade my VE from version 10.2.4 to version 11.5.2.0

Manipulating the contents of the UCS archive is one way to accomplish any necessary changes, and I suspect it is the most common.

The bigip_base.conf file contains configuration for the base system. You can think of it as the elements that are not specific to a module. It includes things such as VLAN configurations, non-floating self-IPs, link-aggregation trunks, and so forth. On 10.x, the LTM configuration elements are in bigip.conf and the GTM configuration elements are spread across files in /config/gtm.

I'd recommend trying to simply install the UCS, without changes, on an 11.5.3 system. If you are using an appliance and can only perform the upgrade during a maintenance window, you can reach out to your F5 Account team and ask for a VE eval license. Because a VE generally only has three tmos interfaces (1.1, 1.2 and 1.3), you may need to modify bigip_base.conf and change references to any other interfaces (I generally just set them all to 1.1). If the UCS fails to load, /var/log/ltm should have information about why the load failed and potentially, remediation pointers. If you do successfully load it on a VE, you can take a UCS snapshot of the successful load. If you later need to move it back to an appliance, you can "fix up" the interface references after it has loaded on the appliance (this is why changing all interface references to 1.1 can be helpful. All VEs and appliances have a 1.1 interface).