Handbrake is a free open source video transcoder which you can use for this purpose.
After downloading and installing Handrbrake open the programme and follow these steps to compreess your video.
When Handbrake opens, press ‘Source’:
Choose where you would like the compressed file to go by pressing ‘Browse’:
…choose the output format, MP4 works nicely:
Choose the quality you want to reduce the file below, in this case I’ll reduce by around half, so I put the slider half way:
…once everything’s ready, press ‘Start’ and the file will be compressed: