How to add formula field or create custom action for particular criteria

How to add formula field or create custom action for particular criteria

Dear Team,
We have "Complaint submit Date" Date field and "complaint priority" Drop down field (High,Medium,Low)
Now we want to create "DueDate" Date field with some function or formula. if we select "Complaint priority" filed is "High" then "DueDate" field will update with "Complaint submit Date" + 60 days.

Example: "Complaint submit Date = 7th-April-2020" and "complaint priority" = "high" then "DueDate" = 6th-June-2020

Please suggest the steps to create the field as per above criteria.

Thank you in advance.