How to extract text from video
Automated text extraction from video is still an emerging technology.

There is a Microsoft public preview of Azure Media OCR that will attempt optical character recognition at a configurable sampling rate. Using this tool requires programming knowledge.

Another option is an open source proof of concept from a research paper, LOOV. You will need to compile this from source.

This post describes a workflow for automatically extracting screenshots to stills and then running OCR on the stills. You could probably adapt this workflow to other tools besides what are described in the post.