What issues are you having with them? Don't they play on certain devices?
Some more details would help: Like audio & video codec or which device was used to record / create them.
If it's only for the sake of 'modern format', you will only decrease the quality by converting them (unless some AI upscaling is involved).
If it's about actual codec incompatibility, a commonly used tool to convert video files is Handbrake.
HandBrake is an open-source, GPL-licensed, multiplatform, multithreaded video transcoder.
handbrake.fr