This site is specifically targeted to software engineers, so it's a reasonable assumption that we can talk about the relevant topics without having to simplify things for people outside that group.
For a reverse proxy, you would want a computer capable of the encryption methods the modern web's security standards demand and to install a web server on it which you can access from the older computer. The server computer does not have to be fancy at all. A Raspberry Pi can do it. For software I would recommend either Caddy (https://caddyserver.com/docs/quick-starts/reverse-proxy) or NGINX (https://docs.nginx.com/nginx/admin-guide/web-server/reverse-...). It can be rather complicated and difficult for someone to do, especially if you're not a web programmer.
If both that and upgrading your computer aren't in the cards at the moment, I would think using the recently-discontinued browser Classilla is your best bet: http://www.floodgap.com/software/classilla/ It at least supports some deprecated forms of TLS & SSL. I hear there's a fork called Phoenix that kind of supports TLS 1.2 even.
(I would recommend using the most updated browser regardless.)
