Click 'File Browser'
On the gallery page search box input the original file name and select the 'filename' result
On the video file page click 'More'
On the video page click the 'Usage' button - all the sequences that contain this video will be listed.
Click 'Select Replacement Video'
Click 'Select' in the 'Select Video' box
On the gallery page search box input the replacement video file name and select the file name result
On the video file page click 'Select'
This will take you to a page that lists the sequences that will contain the replacement video
Click 'Replace'
The original and replacement video files will be displayed along with the list of usage.
Check the details and if you're happy to proceed check the 'I wish to replace this video in x sequence instances' box and click 'Replace
This will take you to the 'Details' page of the 'replacement' video. Click the 'Usage' button to show a list of sequences that now use the file
To confirm the replacement find the old video file in 'File Browser' and on the 'Details' page - the 'Total Usage' should be 0
You can also find one of the sequences and check that that file has been replaced