iPhone Settings > Safari > Default Browser > [Preferred Browser].Its not a Memmy thing, its a system-choice.

Should work for ya

Edit: Oh, you mean like "in-app". I thought in-app was ALWAYS Safari but I sort of remember being able to use alternative browsers in Apollo, although I feel like it was out-app rather than in. Hmmm

Edit: I think its a good practice to not view things in-app if privacy is a concern. I don't think its an issue with Memmy but there was a lot of hackles raised (rightfully so) that many apps like Facebook were abusing in-app browsing where people didn't expect to be tracked/datamined, especially if logging into personal accounts etc.