You can't. Sadly, the iPad version seems to just be a lazy copy of the landscape interface from the iPhone version, which also means you therefore can't access any function that only shows up in the iPhone portrait interface, such as the playback timeline filters - a pretty major new feature! Typical of Hikvision to first unnecessarily lock the orientation on iPad, or not bother to copy the portrait UI elements (since forever), but then introduce a major new feature but only half complete the UI for it. It's like they could't think where to put the filter controls in the landscape UI, so just didn't bother.
In the past they've tweaked around it by, for example, adding left/right buttons at the ends of the date/time display above the playback timeline to allow going forward and back a day, and also allowing you to click on the date/time to bring up a date picker. Neither of these then made it back into the iPhone version, so you have to always flip back to the portrait view to chance date. Taping the timeline to temporarily bring up the filter selection would be an obvious way to do it without permanently wasting UI space, but such an innovation seems out of reach for the Hikvision software team.
On the iPhone version , they added a button to switch from between landscape/portrait (available in both views!), but no such button exists in the iPad version. I say iPad version, I'm not sure they're even a different app package. They certainly don't show up as separate apps on the App Store. If they did, you could just install the iPhone version and problem solved. It's like Hikvision have done everything they could to make the certain features unusable.