You don't. After you get the lantern and the footprint photo, there is no other proof available in the Himalayas. Once you prove the other three (Nessie, Chupacabra, and Jersey Devil), an opportunity will come up to chase Bigfoot instead of the Yeti, since they are similar creatures.
There are 3 you have to prove: Nessie (Loch Ness) Chupacabra (Puerto Rico) New Jersey Devil (New Jersey) You can't get what you need to prove the Yeti, so you are sent to find Bigfoot.
You still have to prove the chupacabra and the Jersey Devil. You can get a photo from the Himalayas, but it does not prove the Yeti. So when you have the other three proven, you will be sent to chase Bigfoot in the Northwest.
After you have the chupacabra tooth, you still need to prove that Nessie and the Jersey Devil exist. (The monks are stubborn about the Yeti and your snow footprint is unconvincing.) Get the photo of Nessie, and some egg shells from the Jersey Devil, and then you can pursue Bigfoot instead of the Yeti. (see related questions)
Prove 4 cryptids to qualify for the Mews million-dollar reward. You find a footprint that may be from the Yeti, but you can only prove Nessie, the Chupacabra, and the New Jersey Devil. Then you have to track Bigfoot and save him from Gretchen Grimlock. (see the related solution question)
You will search for evidence in four locations around the globe where cryptids have been observed. Ask you discover possible clues, you examine them to see if they are genuine. Nessie - at Loch Ness Yeti - in the Himalayas Chupacabra - in Puerto Rico Jersey Devil - in New Jersey You will be able to prove all but the Yeti. Only when you prove them, and have the unusable footprint photo from the Himalayas, will Mews send you in pursuit of Bigfoot in the Pacific Northwest.
Since the monks won't let you test the Yeti scalp, you have no proof. Once you have the lantern and the footprint photo, you can find nothing at the Himalayas. However, once you prove the other three cryptids (Nessie, Chupacabra, and the Jersey Devil), Mews will send you in pursuit of the similar creature, Bigfoot.
After you only get a footprint photo from the Himalayas, Mews decides that it is too difficult to track the Yeti. When you prove the other 3 cryptids, he sends you in search of Bigfoot, and you have to rescue the creature from the evil Gretchen Grimlock.
You cannot prove the Yeti based on the evidence from the Himalayas. So you have to prove the other 3 (Nessie, Chupacabra, and New Jersey Devil) and when you do, Mews will send you to pursue Bigfoot in the Pacific Northwest.
Once you have gotten the lantern and the footprint photo from the Himalayas, you still cannot prove the Yeti. You can prove the other three cryptids (Nessie, the chupacabra, the Jersey Devil). Once you do, a fifth location will appear on the helicopter controls: you must go to the Northwest to chase Bigfoot.
You cannot prove the Yeti. But after you confirm the other three cryptids (Nessie, Chupacabra, and Jersey Devil), Mews will send you in pursuit of the similar creature, Bigfoot, in the Pacific Northwest. The location will be added to the GPS on the helicopter.
The snow footprint looks more like a snowshoe than a Yeti, so you have to give up on the Himalayas. (Get the lantern from the monastery.) Once you prove the other three cryptids (Nessie, Chupacabra, and Jersey Devil), Mews will send you to the Pacific Northwest in ssearch of Bigfoot, which is a similar creature to the Yeti.
After you have confirmed the other three cryptids (Nessie, Chupacabra, and the Jersey Devil), and brought back the inconclusive footprint from the Himalayas, Mews will send you in pursuit of Bigfoot, which is similar to the Yeti. Since you have him in a cage at the end (but let him out), he is definitely confirmed.