The sole owner of an LLC owns 100% of the membership interests of that LLC. What you require is called an assignment, which is a document that transfers the membership interests to you and states the consideration (what you paid in cash or kind) for the interests. You should also check the laws of your state to determine if it requires notice of this change to be filed with the state. Some states require that the names of the owners be listed on the articles of organization, in which case, an amendment to the articles would also be required. You should consult a local attorney familiar with these transactions to prepare the proper documentation.
Creating a MAP will take no more than an hour of your time every month and will keep the lines of communication open, ensuring relationships with investors remain strong, and ultimately helping early-stage startups succeed.