I need to change name of an app, but for that i need to upload a new build. I no longer have the app source.
It is theoretically possible if you have a build installed on a physical device. You would need to make a backup to iTunes on your computer, manually find the IPA file, extract it (its a Zip file), change the information you want, then re-sign it with a new provisioning profile then re-upload via Applications Loader to submit to Apple for review.
Note that this will likely not update any reference of the app name inside of the app unless you properly localized the app, in which case you may be lucky and be able to update the localized strings files.
Note that even after doing all this, your application is likely to be rejected by iTunes as various requirements have changed throughout the years: marketing icon, camera usage description, app transport security, export compliance, etc..
In short, you should have made a backup of your sources. Good luck though!