How can I merge two faces to become rounded? (more details in description).

Hi all, I'm relatively new to onshape and am working on a project for a friend. It involves very precise measurements and tiny details and I have been trying to complete one of these key details but as of yet my efforts have borne no fruit. Essentially, I am trying to merge these two faces, creating a smooth, rounded, transition between them (I suppose in some ways like revolving it). The CAD file, as well as the product I'm trying to copy, is shown in the picture below. Any help would be appreciated - I know the phrasing is a bit unclear, can't think of any way to say it otherwise.

 https://cad.onshape.com/documents/c88b344060fac3f5718edd54/w/d145594edd898ca4f77ef2c9/e/94cbf1f9b7b1116193d5527b?renderMode=0&uiState=686ad47e44ca860aefe48106