The best way to keep your weight up to date in Nico is to use smart scales that automatically update your weight. For example, Withings has a range of smart scales, but any that connect to Google or Apple Health will work.
Adding weight manually
You can't add weight to the Nico app.
If you don't have smart scales, you can add your weight to Apple Health or Google Health, and Nico will import this (as long as you've connected this service).
Apple Health
Open the Health app on your iPhone.
Tap the Browse tab at the bottom right, then tap Body Measurements.
Select Weight.
Tap Add Data or the plus (+) icon in the top right corner.
Google Health
On your Android phone, open the Google Fit app
At the bottom right, tap Add
Choose add weight
Enter your weight.