Log into your AdminCP and go to:
Settings >> System Settings >> Manage Settings >> User
Look for the setting: Check For Promotions Enable it and save your changes.
then go to: Users –> Promotions –> Add Promotion
User Group: Select the user group for which you want to create a promotion for.
Activity Points: This is how many activity points a user must get before they will automatically be promoted to another user group.
Days Registered: Show the number of days the user is registered before they will automatically be promoted to another user group.
Move User to User Group: When users have reached the activity point and days registered you have set they will be promoted to the user group.