Clarify the difference in EF between an alternate key and a unique index

I followed this but I cannot update the License Plate because it is a key.
How to make it unique but we still able to correct the License Plate later ?

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#### Document Details

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* ID: 19fb72e5-5309-7b90-a95d-514a2c62c50c
* Version Independent ID: 1d7d65a5-f8cb-cd45-3dff-34a87da0df2d
* Content: [Alternate Keys (Unique Constraints) - EF Core](https://docs.microsoft.com/en-us/ef/core/modeling/relational/unique-constraints)
* Content Source: [entity-framework/core/modeling/relational/unique-constraints.md](https://github.com/aspnet/EntityFramework.Docs/blob/master/entity-framework/core/modeling/relational/unique-constraints.md)
* Product: **entity-framework**
* GitHub Login: @rowanmiller
* Microsoft Alias: **divega**