You have to update your Default Templates. If you have one that you have already created for your assembly templates you need to tell Solidworks where to locate it. Ensure that your Document Templates have been located first so that you have search locations for your Default Templates. Options>System Options>File Locations>Document Templates in the drop down menu. Then when you've located that you can go to Options>System Options>Default Templates and select your assembly template.
You have to update your Default Templates. If you have one that you have already created for your assembly templates you need to tell Solidworks where to locate it. Ensure that your Document Templates have been located first so that you have search locations for your Default Templates. Options>System Options>File Locations>Document Templates in the drop down menu. Then when you've located that you can go to Options>System Options>Default Templates and select your assembly template.