Well it is up to you if you want to upgrade or not.
Upgrading from El Capitan to Catalina is a big jump and includes
the reformatting from the old HFS+ file system to the new APFS file system.
Some have reported the installation stalling when upgrading from the older OS's.
I would recommend upgrading to macOS High Sierra first.
Here is what to do,
Open Safari and click on this link,
How to get old versions of macOS – Apple Support
Then go to the Download OS section and click on the OS you want to install.
Firstly High Sierra and then Catalina.
Before upgrading make a full back up of your Mac.
Check that third party apps you use have updates available for the new OS.
https://roaringapps.com
Check that your printers/ scanners etc have updated drivers and software available for the new OS.
Connect your mac to your router via cable rather than rely on WiFi.
Do not use your mac while it is downloading
and do not use any other devices that share the same internet connection.
If the App Store says ‘Open’ then it is detecting a previous download of the install app
go to the Applications folder locate Install macOS ******.app.
Move it to the Trash, empty the Trash.
Restart, click on the link again, the App Store should say ‘Get' or 'Download'.
If you are considering upgrading to Catalina or Big Sur be warned they do not support 32bit apps.
Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,
some updates may be free, some may be paid. Use this app to discover which apps you have
are 32bit, https://www.stclairsoft.com/Go64/ then contact the developers of the software to see
if they are providing 64bit updates or alternatives.