1) After you download it, extract it with WinRAR on Win / Archive App on MAC, and open the .html file.
If it redirects somewhere else and you see a different landing page, this is probably an "anti spying" protection, which needs to be removed.
Usually it looks like this:
You can simply remove it or comment it temporary while editing out lander, but it's good to have it while you run traffic.
2) Next we need to replace CTA (Call To Action) links (so they point to your tracking link) on the landing. Easiest way is to open .html in notepad and replace every single link you see out there.
3) Inspect JS code (also .js files in the folder), if you see words like "eval" . This is a major red flag that this lander might be backdoored.
Usually it's safe to replace jquery.js files with legit ones from Google, because "smart" affiliates usually put a backdoor in there.
To make sure the lander is not backdoored open Chrome developer console (Ctrl + Shift + I) and open Network tab. Refresh page 10-15 times and if you see any request to domain that's not yours, lander is probably backdoored.
There are no general rules how every backdoor might look like or how to remove it, so proceed with caution.
If you don't know JS, ask someone to inspect it for you.
The safest way is to get the landing page re-coded from scratch.