Exact details depend on your browser.
One method is to go here:
https://www.whatismybrowser.com/
You should see some information about your current browser, and either a green box at the top that says "Your browser is up to date" or else a red box that says "Your browser is out of date" which may have a link with details about updating.
Take care not to click on any of the ads on that page, though.
Here is some info about supported browsers for eBay (though I think that Safari is mistakenly included among the Windows browsers), with links to get recent versions of each browser:
https://www.ebay.com/help/policies/technical-issues/technical-issues?id=4220
