what can i do now that my iphone6 plus is not compatible for latest ios

Purchase a newer iPhone that can still receive updates. At least an iPhone 8 although you would want to get the newest one you can so that you know you will have more time with it still being supported by iOS updates in the future.

Unfortunately that’s the only option you have. You can continue to use that iPhone but as you probably have noticed, a lot of your apps are going to stop working because they will require you to have a higher iOS for them to continue to work.