By default elgg is not packed with push notifications. If you are seeing them in your site, some plugin or theme you are using is causing this. 

I have removed sw.js file from root folder . Please check again on all devices .

No. We don't need to check anything. YOU should check what's sending these push notification on your Elgg site. As Sarath said, Elgg sends no push notification without any additional plugin installed that provides this functionality. Disable this plugin and the push notifications should stop. I don't know what the sw.js file should be. But removing any files without knowing their purpose can easily break the site. So, you might think before installing any stuff you don't need and then asking each day a new question about something not working on your site.

Are you talking about elgg or the plugins? If you are referring elgg then all elgg version are upgradable.

• Upgrading 1.x to 1.y - Here
• Upgrading 1 to 2 - Here
• Upgrading 2.x to 2.y - Here
• Upgrading 2 to 3 - Here

You have all the options. 

On Elgg 1.x you would have to upgrade only one minor Elgg version (2nd number) at a time, i.e. 1,7 -> 1.8 -> 1.9 -> 1.10 -> 1.11 ->1.12. With each minor release you can update directly to the latest bugfix release (the highest 3rd number).

Starting with 1.12 it gets easier. From Elgg 1.12 you can directly upgrade to the latest 2.3 release and from the to the latest 3.3 release.

Upgrading Elgg is only one part of the upgrade process if you have any 3rd party plugins installed. If you use any additional plugins you also need to have compatible releases of these plugins. Best would be to upgrade them in a manner corresponding to upgrade Elgg. With Elgg 1.x the plugin compatibility is not easy to know in general (depends on which plugins you use). Starting with Elgg 2 a plugin release for Elgg 2.0 should still work up to Elgg 2.3 and the same for Elgg 3.

If you can upgrade now already depends on which plugins you use. You also need to keep in mind that server requirements change with newer Elgg releases. For example, up to Elgg 2.3 you can use only PHP 5. With Elgg 2.3 both PHP 5 and 7 work. With Elgg 3 you must use PHP 7 (7.2 at least with latest Elgg releases). MySQL might also need to be upgraded in between upgrading from Elgg 1 to 3. Read the docs with regards to the requirements listed for each Elgg release. So, you not only need to plan the steps of upgrading Elgg itself but also when to upgrade you server.

It might seem difficult but the better you plan in advance (and even better test the whole process beforehand for example on a local test installation) the easier it is. Always do backups as often as possible (i.e. on every intermediate upgrade step at least). Then it's even less risky.