To convert a String or Number field to Formula field in Salesforce, you can do the following workaround.

We cannot convert String or Number field to Formula field in Salesforce.

Workaround: 

1. Create a temporary field and use the temporary field in all the Visual pages, Classes, Triggers, Components, etc. where it is referred.

2. Delete the Number or String field.

3. Create a formula field.

4. Now use the created formula field in all the Visual pages, Classes, Triggers, Components, etc.

Cheers!!!