In Classic, on the account tab, there was a very handy “account merge” button in the bottom right of the page. This did not come along with Lightning unfortunately. To this day, there is no out-of-the-box way to merge two accounts in Salesforce. For nonprofits users, this may not be a huge deal. But duplicates occur all the time for us as we track affiliations. So I began searching for a solution a few months after Lightning arrived.
The official answer from Salesforce is that you don’t need it. Instead you should enable duplicate checking rules and then you can merge using the Potential Duplicates component. This only works if the accounts are already detected as duplicates. Bulk duplicate checking is only available for customers with an unlimited license. This doesn’t apply to Power of Ten license holders, and it’s a lousy response anyway IMO because I can do things in classic to merge things that will never likely be detected as an account duplicate.
I sought to bring the classic function to Lightning outside of any official support. The only success I have found to date is to a hacked link to the lighting utility bar. It opens a new window (in classic styling) and allows the merge. Then you just close the window. It’s not ideal but it works better than any solution provided by Salesforce so far.
Here is it on the utility bar. It’s the last one. The link is mydomain specific so you’ll have to change that part. (I couldn’t get a shortcut version to reliably work.) We tried a lot of places but only found the utility bar to be trustworthy. You might be able to get the link to work in a rich text component on a home page if needed or possibly some lighting page components designed to share shortcut links. They didn’t work when we first released this hack internally about a year ago.
It’s not pretty but it does save switching to classic to do the merge.