Error: Function ISCHANGED may not be used in this type of formula - how to resolve this?

To resolve the Error: Function ISCHANGED may not be used in this type of formula -

You can only use ISCHANGED in the following cases:
Assignment rules
Validation rules
Field updates
Workflow rules if the evaluation criteria is set to Evaluate the rule when a record is: created, and every time it’s edited .
Formula criteria for executing actions in the Process Builder.
To use ISCHANGED in a Workflow Rule, use the "when the following formula evaluates to true" option.