At Backup creation the Temp folder will default to C:\Users\(usersname)\Temp. However, if an alternate location was chosen it can be found through the following process.
1. Select Backup sets in the UI
2. Click on the Backup set itself.
3. Select Show Advanced Settings.
4. Identify the path of the Temp folder.
5. Once you know the location you can navigate to it using file explorer and delete the contents. Additionally you have the option to change the folder location. This is especially helpful for errors indicating "There is not enough space on disk". In this case you can direct it to a different drive or partition with more available free space.