Hi @Yeasin,
Indeed, when you upgrade a supported AKS cluster, minor versions can't be skipped. For example, upgrades 1.14.x -> 1.15.x or 1.15.x -> 1.16.x are allowed, while 1.14.x -> 1.16.x is not allowed.
(!) However, skipping multiple versions can be done when upgrading from an unsupported version back to a supported version. This is your case as well since 1.21 is an unsupported version, so you can upgrade directly to any of the currently supported Kubernetes versions from the list.
To check the currently supported Kubernetes versions for your AKS cluster and region by running:
az aks get-versions -l <region>
What you need to be careful of is that many things have changed between 1.21 and 1.25 and there might be inconsistencies in your cluster configuration or application configuration so you check in advance if your application works fine on 1.25.
Version 1.25 also runs Ubuntu 22.04 by default, while earlier versions are with 18.04
You can also test everything in a new cluster with 1.25 on which to deploy your applications and configurations if you want to keep the old cluster for your workloads and just upgrade the version.
Hope this helps! Please be sure to accept the answer if this was helpful and let us know in the comments if there are any follow-up questions.