To access the classic site version of eBay you need to use a browser. Go to ebay.com and scroll to the bottom of the screen and use the "classic site" link there.
If you have difficulty with the classic site reverting back to the mobile site, try requesting the desktop site version of the page instead:
https://community.ebay.com/t5/Technical-Issues/Will-not-stay-in-Classic-Site-mode-listings-appear-in...
If you open links in the same tab they should stay in desktop mode, but if you open another tab you will have to request the desktop site again in the new tab.
If you have problems with eBay browser links opening up in the app, see here:
https://community.ebay.com/t5/Member-To-Member-Support/new-page-opens-when-I-click-on-a-listing/qaq-...
If you have problems with the eBay app, make sure you have the latest version that works with your OS:
https://community.ebay.com/t5/Technical-Issues/App-for-tablet/m-p/31024025/highlight/true#M72111
