First you need to create a logo with the following ideal dimensions: 280px by 80px. If you do not have any image editing software then you can use a free online program to resize any image: https://pixlr.com/
Make sure you remember the file location where you save the final logo.
Navigate to Dashboard > BuddyBoss > OneSocial Theme > Logo
Select the "On" switch under Desktop Logo. Then select upload and navigate to that file location you remember. Finally select open and remember to SAVE your changes.