I was wondering if updating a server to a new version would erase the whole world, and if it does, is there any way to update it and only erase the nether (for the nether update). If none of that works is there a way to save the over-world and update it, then import the over-world as to not delete any content? Thanks.
If you switch only version, and not software then the world should stay. If you switch version and software, this will depend on the software you're switching to.
If the button only says "install", it shouldn't erase your world.
If the button says "reinstall" it will.
Always make backups before switching.
For the nether world, you'll have to delete the nether world in https://aternos.org/worlds/. Either its straight away on that page or you'll have to dig around in your main world's files, and delete the dim-1 file iirc