When my apple Niky watch has been on charge all night the curve doesn’t last the whole day. How can I train my apple watch battery last all day?

Your post suggests you are using Apple Watch Series 2 which is probably about 5 years old and the battery is the same age. Apple generally expects its batteries to retain at least 80% of their original performance after 500 charge/discharge cycles and I am pretty sure you are well past that point.

However your Apple Watch Series 2 is still supported and you can put it in to Apple for battery service. Please try (link) Apple Watch Service & Repair - Apple Support where you will be able to get an estimated cost. (If that link doesn't come up with your local currency you should be able to search again and find it, otherwise use https://getsupport.apple.com/ and work your way through the options).

Please also be aware the battery service for Apple Watch is only available direct from Apple or Apple's Approved Service Centers. A street corner independent or unofficial repair will be a scam.

Apple US says it's $79 to replace the battery in your 7 year old watch. (Follow the link Branta provided to get the estimate).