What program do use use to convert

What program can I use to change MKV shows from WebDL 720p to WebDL 480p pr from HDTV-720p to SDTV
sometimes I miss a episode of a tv show because I cant find the smaller size and have to download a larger format.
What format is the most popular for keeping your shows

Well being that Harddrives aren’t that expensive most people download at least 720p versions,
It isn’t worth the time and hassle to convert them to a lower quality.

I prefer a higher resolution and quality and at least 5.1 audio,

I used to collect the 1080p iTunes WEB-DL releases,
But switched to the 1080p Amazon WEB-DL releases.
The Amazon releases are superieur to the iTunes releases.