Replace existing modem

Hello! 
I have been having a lot of intermittent connectivity issues with various devices all dropping at different times. I have talked too representatives about this but it seemed to work when they were in the call. I asked my neighbor about it and they had the same problem.  The modem we have is an older version. What is the process to try and replace this at one of the Xfinity stores? Do I need to contact support first, or can I bring it in somewhere?

We are in the middle of exam season and would like a replacement soon.

Thanks!