2006 starting issues

I replaced the battery on my 2006; the new battery worked fine for about 4-6 weeks. Now when attempting to start the truck, I get the dreaded clicks, not turning over, but headlights come on okay. Could not lock the doors with the remote, nor could all doors be locked from driver switch (remote has new battery). I had to lock all doors manually. And about two weeks ago, could not get passenger window to go up. It took a few attempts with the switches, both driver and passenger side, and turning starter switch on/off a few times.

Help is much appreciated, and thank you.

 

Update: right after creating this thread, I checked batt voltage. It is at 8.5vdc. So now assume alternator is the culprit. Any ideas on how to test alternator? Or pull it and take it to shop for testing, or just replace it?

Thanks kindly.