I’ve try many times to install iOS beta 14 but still couldn’t..plz help how to install it

Sorry, but we can't assist you with any issues related to Apple beta products on this forum. If this is a developer beta, you would already know that Apple has provided you with a means to contact them. On the other hand, if this is a public beta, Apple does not provide any form of support and you use these betas at your own risk.

Your 8 1/2 year old iPhone 6 Plus hardware is not capable of running any iOS version higher than 12.5.7.