The next step depends on what you want to achieve. Do you want to update the package for future use by you and/or others? Then the cleanest way would be to put a patch-file into "srcpkgs/<packagename>/patches/". Then increase the revision in "srcpkgs/<packagename>/template". This is of course the tricky part. Afterwards you can build an dinstall the package as you have written. Just add the package name at the end of your xbps-install line :)

If you only want to make the changes this one time and not preserve them you can get xbps-src to get the sources for you:

Edit the the sources as desired in "masterdir/builddir/<packagename+version>/"

Build and install like this: