wnr2000-100NAS*. How do I install new firmware?

I have had unit a long time. Today I have Xfinity XB3. I dropped Xfinity TV and kept internet. I have 3 floors: A basement, middle floor and upstairs. When I dropped Xfinity, I went to Roku TV and it works everywhere. I noticed the Wireless Signal was weak in the basement and upstairs. I had this older WNR2000-100NAS* (which is on the box) forever. I youtubed a saw that I could create a wireless extender. It worked perfectly. My basement had a really slow signal. I followed youtube on how to get into the router. I had a hardwire in the basement available from my main Xfinity Modem Router. I set up the WNR in the basement, and now I have a great signal for 3 TVs and anything else I want to use in the basement. Problem solved in basement.

I have two questions I need help on:

1. I am sure this unit is at least 10 years old.  How do I upgrade the firmware and is it available?

2. I have 3 floors. The main Xfinity Modem Router is on the middle floor. I have solved the issue in my basement with the older WNR2000.  I have a hardwire available on the top floor. I want to do the same thing on the top floor. I assume I can do that since I have a good internet hardwire connection upstairs to work with. I see that that are all kinds of WNR2000's, (V1, V2 etc...) what is the most current model in that category and I assume I can add a 2nd one. I am sure there is someone out there that has done this before.