how to downgrade whatsapp on iphone

I use whatsapp for work and use the computer version a lot. I recently accidentally upgraded my version of whatsapp on my iphone and now it is not compatible with the computer version because I have an old operating system on my laptop. I am running Yosemite 10.10.5 and cannot upgrade that as I am still using Final Cut Pro 7 and it won't work if I upgrade.

I tried downloading apptrans (after much googling) but that wouldn't work on my old IOS and my iTunes is too new 12.7 for the other technique I found online.

I just need to go back to my previous version of whatsapp on my phone so it will be compatible with the computer version.

If anyone can help me, I would be most grateful as it has become very difficult to work like this using only my phone. I keep having to email myself things or copy and paste text to complete my work via my phone.

Thanks in advance for any guidance.