If a place does not have a 'solo' button you cannot play this place on your own. Don't try and find a way because unless you are on your own in a server there is no possible way to play solo.
You can edit models on Roblox by ungrouping them and then regrouping them later. To do this, select the model, and then hit the ungroup button. Edit the model as much as you like, and then select all...