Addons that have been abandoned

There are many great addons that have been abandoned and no newer version has come or the user is no longer active in the community.
Is it allowed to download the addon make modifications to make it more up to date and then upload it by myself as new one? And how should the credit given to previous user who has done it?

That is sad but true that sometimes it happens so. I think correct approach, which is applicable in open source world generally is to fork the project in GitHub (almost all add-ons have the repository there) and approach the author of the add-on with a pull request. In most cases authors of add-ons are favourable.

I know that sometimes ownerships of the open source projects have been transfered to new more enthusiastic hands. New projects have been created based on old ones too. Check the projects license file for possible restrictions. It is a best practice to mention all the prior art in readme and license notice. Also remember to check that license types match, preferably continue to use the same open source license as the original one.