LLP does not have shares. What you can buy and sell is called as partnership stake.
You have to execute a admission cum retirement deed which will lay down the terms of the exit. It will mention the consideration, the terms of admission for new partner , the amount and mode of the capital to be brought in, the duties and rights of new partner etc. It will also contain the terms of payment to the retiring partner. It will contain the dates of the admission cum retirement also.
Please note all this is subject to the LLP Act, 2008 and the LLP Rules,2009 and the subsequent rules. the deed cannot overwrite the statutory rules in any case.
For example, the deed requires minimum of 2 partners at all times. Hence if there are only 2 partners in a LLP, the admission has to occur on or before the same date as retirement. Admission cannot occur after the retirement of the existing partner.
Further, appropriate stamp duty has to be paid on the deed, depending on the assets brought in.