2. Modifications to this Agreement<p> Tumblr reserves the right to modify this Agreement by (1) posting a revised Agreement on and/or through the Services and (2) providing notice to you that this Agreement has changed, generally via email where practicable, and otherwise through the Services (such as through a notification on your Tumblr Dashboard or in our mobile applications). Modifications will not apply retroactively. You are responsible for reviewing and becoming familiar with any modifications to this Agreement. </p> <p> We may sometimes ask you to review and to explicitly agree to (or reject) a revised version of this Agreement. In such cases, modifications will be effective at the time of your agreement to the modified version of this Agreement. If you do not agree at that time, you are not permitted to use the Services. </p> <p> In cases where we do not ask for your explicit agreement to a modified version of this Agreement, but otherwise provide notice as set forth above, the modified version of this Agreement will become effective fourteen days after we have posted the modified Agreement and provided you notification of the modifications. Your use of the Services following that period constitutes your acceptance of the terms and conditions of this Agreement as modified. If you do not agree to the modifications, you are not permitted to use, and should discontinue your use of, the Services. </p> <p> Note that, if you have prepaid for any Paid Services (as defined below) prior to a modification of this Agreement, your use of such prepaid Paid Services is governed by the version of this Agreement in effect at the time Tumblr received your prepayment.

They can change the terms of service any time they see fit, but they will notify the user. Your use of the service supposedly constitutes acceptance of the changes in the terms.