On a windows machine, the replaced version is moved to the recycle bin. You can drag it out, move it back into the apps folder or add it to your library using itunes. Move or delete the new version first.
If you're on a mac, check your Trash - the old ipa file might still be in there. Do you use Time Machine to back up your files? If so, you should be able to get the old version back that way. If you're on a Windows PC, you could do a system restore to the day before you updated the app.