You can't change the type of custom data it contains. If it was created as a single line text field, it's stuck that way. If you have the need to show more data in that field consistently, you could create a new field as a multi-line text field and copy the data from the old field to the new field. (you'd probably need a tool to do this, like ScriptRunner)
Hello Community! My name is Brittany Joiner and I am a Trello enthusiast and Atlassian Community Leader. I'll be sharing with you a series of videos with my top tips on how to best use Trello...
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