How to install apps from Sd card in win 10.
That's easier than WP8.1. But you'll need to download the xap/appx files from a third party, but legitimate, source such as Appx4Fun - they give you original and untouched WP apps. It's better to download from their Microsoft CDN server as it's directly sourced from the Windows Store. Note that you cannot install patched apps by this method, or may be you can. Anyways, try if you can.
Then enable Sideloading apps or Developer mode by going to Settings > Update and Security > For developers.
Next you'll have to copy/move the downloaded xap/appx file on to your SD card (location doesn't matter). Then navigate to that location and tap the xap/appx file (it has no icon; just a generic one). It'll ask you if you want to install it. Click yes and your app will be installed silently in the background, no progress dialogue will be shown. After few seconds check your All Apps list (the time required to install the app may vary with app size). Your app will be shown in the Recently added section.
This method is applicable to phone storage also. With W10M, you can install apps from any storage.
Else, the usual method by using Application Deployment tool in the W10M SDK is an other option.
Psst! If you are talking about Windows 10 PC, then it's harder. First of all you'll need the app file which is very hard to get. If you do manage to get them then there's a Win32 program called as Metro Sideloader. It'll help you further. You'll need to enable sideloading apps on PC the same way as on mobile. But I am pretty sure you meant Windows 10 Mobile.