Firstly you will need to upload the image into the image gallery:
Utilities > Documents > Listing > Actions > Add
Add in your image details by entering in the Title, Type, Select File > Save
Once that is saved copy only the red part of the URL
Secondly you will need to replace the current asset withe new asset you have just created.
To do this you will need to go into your Shop tab and follow the instructions below.
Shop > Shop Settings > Design > Homepage Tiles > Actions > Edit
Once you are in Edit mode you will need to select SOURCE at the top left hand corner and locate the current asset code and paste in the new code.
Shop > Shop Settings > Design > Homepage Tiles > Actions > Edit > Source > Find current asset > Paste over with new asset > Source (to view if it is has been updated correctly) > SAVE.
Tip: Once you have updated the Asset ensure it has been done correctly by clicking SOURCE after you have pasted over with new asset.
Tip 2: If you are changing homepage tiles and they link to a new locations, ensure you updated the link in the code.