I can't seem to be able update IOS12.5.5 to IOS13 or above for my iphone6, even tho I'm on auto-update.. Help!

My Yahoo Email quit being received on my cell phone about 3-4 weeks ago. Can you help me find a way to update my iphone 6 from IOS 12.5.5 to IOS 13 or newer? I tried unsuccessfully for several hours this past week. Even tech support from ATT and Yahoo couldn't help, nor Apple tech support. I heard Apple may have dropped their support to iphone 6 and older, but I haven't been able to confirm that.