From time to time we need to update our database version to latest to get the new changes. it could be a major change or minor change.
For minor changes, AWS has provided a option "Auto Minor Version Upgrade" feature for Single RDS, Multi AZ & Aurora single Cluster.
But that feature doesn't apply to the following kinds of Aurora Global clusters:

Clusters that are part of an Aurora global database
Clusters that have cross-Region replicas

So, we need to follow some process to update that global DB.

First, Remove all secondary Regions from the global cluster.

Just Select the secondary cluster and go to “Actions“
So, that secondary cluster will be a regional cluster and a seperate cluster.

Upgrade the engine version of the primary Region to desired version, as applicable from Portal or CLI or Terraform[any IAC]

Now, if "Enable Deletion Protection" is enabled then please disable that option for secondary cluster.
Now delete that secondary region cluster/ you could use as another cluster if needed. if you don't delete then either you have to add with another DB identifier or you will get thise error.

After Deletion, please select the Global Database and go to "Actions" and "select Add AWS Region". [check references no-3]
If you use Terraform code then run "terraform apply" and it will take time to create those instances.

How to Update Aurora Global Database Version

References: