How to use the latch in the middle of the handle?

Hello,

It sounds like you might be looking for a patio door handle set with a centered thumb latch. We can't be sure of what you need without seeing the original assembly but you can start by looking at the 82-030. which has an adjustable thumb latch that can be centered or offset. 

Hole center dimensions for 82-030

We also offer the 82-030A keyed version of this handle with the thumb latch fixed in the center position. With that being said, you would need to confirm that the 3-15/16" hole spacing matches yours. Take a look and feel free to get back to us with photos of your hardware to confirm if these parts could work for your application. Thank you! 

Your handle set is a closer style to the 82-089 but, unfortunately, it is not available with a centered thumb latch. With that being said, we recommend the 82-030 to replace this type of handle set all the time. As long as the hole spacing matches up to yours, which it appears to, then you should be good to go.