Today, somehow I deployed react app to GitHub. How can i update that?
Make changes, commit the code to master, then do whatever you just did to build the
gh-pages branch, which while redeploy it. Impossible to advise anything beyond that as have no idea how you’ve deployed anything.
now it says that Browserslist: is outdated , how can i update that
We are not sitting in front of your computer, you need to provide much more context. We have no idea at all what you are doing, what tools you are using, what you are building, how you are building it, we aren’t telepathic.
You might have made changes, so do a pull request then commit and push new changes. After that do like DanCouper said and do the build.