Offline event payment
The 'Contribution status' is automatically updated when the second payment is added. For example:


Register a participant for an event that costs $800
Log the contribution as $400 and save the registration
Note that the contribution is 'Partially paid' in the registration list
View the participant record, click on 'Add payment'
Add a payment of $400
Note that the contribution status has been updated to 'Completed'


If you want to manually change the status of the entire contribution, you can do so by editing the contribution.

If you want to change the status of individual payments then that looks like a feature request! It doesn't appear to be possible to even edit individual payments at the moment.