In oder to do it you need to login to your WHMCS Admin Panel and go to Addons -> VPS Reseller Module

In oder to open custom plans config please follow image below.
Once you open custom plans config you will see this page:
From here you will be able to:
✔️Add new custom plans to products.
✔️Edit existing custom plans.
✔️Delete existing custom plans.
✔️Assign custom plans to specific client.
✔️Configure plans recourse prices.
In oder to add new custom plan you will need to select a product and press on "ADD A NEW PLAN" button
Once you above is done you will that window is extended with input fields:
From here you will need to:
You can edit your plans on the fly.
You just need to look for plan you want to edit and place new values in directly in input fields and press "Save Changes".
You can delete your plans on the fly.
You just need to look for plan you want to delete and press on trash bin on the right side near plan: