When you create the agent rule instance, automatically agent schedule instance gets created. Whatever time interval is there in the agent schedule instance that will get picked at run time of the agent. The preference goes to the agent schedule time instance than the agent rule instance. Even if you modify the time interval in the agent rule form, I think it won't get updated in the agent schedule time instance.
Agent schedule instances were created automatically by the master agent when your agent was created. If you delete the agent schedule instance manually then again agent schedule instance gets created based on the values from the agent rule form.
There are two ways you can follow for searching the Agents.
1. On the Top header of Designer Studio. There will be a Configure option. Click on that and search for Agents. Then click on Records-SysAdmin--Agents and filter with the Rule Set name. Then open the corresponding Agent rule and save the rule in to highest open rule set version. Delete the existing Agent Schedules so that when Master Agents wakes up it will create the schedule automatically.
2.Click on Records Explorer and select SysAdmin category and then select Agents. Search for the Agent and then follow the same above procedure.
But for testing purpose if you want you can directly update the Agent Schedule Instance if there is no open rule set version. This can be done by Navigating to Records->SysAdmin->Agent Schedule. It will open the tab and filter with the Rule Set Name in the Key contains search bar. It will display the Agent Schedule Instance. Update with you changes and Restart the corresponding Agent from SMA, latest changes will be picked up from the next run on-wards.