This is a duplicate from my answer here: https://stackoverflow.com/a/50955293/491553
Here is how I upgrade npm when running nvm-windows:
cd %APPDATA%\nvm\v14.20.0 # or whatever node version you're using
move npm npm-old
move npm.cmd npm-old.cmd
move npx npx-old
move npx.cmd npx-old.cmd
cd node_modules\
move npm npm-old
cd npm-old\bin
node npm-cli.js i -g npm@latest --force
And boom, upgraded.