I assume you just want to lower the capped inlet on the left, without lowering the bottom horizontal drain? Or do you want to lower the standpipe and its trap as well?
If you don't lower the standpipe and trap, then you need to raise your horizontal vent, it's supposed to be 6" above the fixture flood rims, which includes the top of the standpipe. So you could do something like the drawing below, where the straight line segments are pieces of pipe, and the other drawings are fittings.
The lower left is a new wye cut into the horizontal drain, with a street san-tee on the inlet, and a street 45 on top to turn the vent vertical. I didn't draw it in as new, but it's probably easiest to replace the double san-tee and cleanout combo as well, just rebuild everything from the left edge of the photo towards the right.
Cheers, Wayne
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