Canon 401. §1 A diocesan Bishop who has completed his seventy-fifth year of age is requested to offer his resignation from office to the Supreme Pontiff, who, taking all the circumstances into account, will make provision accordingly.

§2 A diocesan Bishop who, because of illness or some other grave reason, has become unsuited for the fulfilment of his office, is earnestly requested to offer his resignation from office.
NB see m.p. Learn to take your leave, 12.II.2018:

Art. 1. Upon reaching 75 years of age, diocesan and eparchial Bishops, and those deemed equivalent to them according to canons 381 §2 cic and 313 cceo, as well as Coadjutor and Auxiliary Bishops or holders of special pastoral responsibilities, are invited to present to the Supreme Pontiff their resignation from pastoral office.

Art. 2. Upon reaching 75 years of age, non-Cardinal Dicastery Heads of the
Roman Curia, Superior Prelates of the Roman Curia and Bishops holding other

offices of the Holy See, do not ipso facto cede their office, but must present their resignation to the Supreme Pontiff.

Art. 3. Likewise, Pontifical Representatives do not ipso facto cede their office upon reaching seventy-five years of age, but in this circumstance must present their resignation to the Supreme Pontiff.

Art. 4. To be effective, resignation pursuant to articles 1-3 must be accepted by the Supreme Pontiff, who will decide by evaluating the concrete circumstances.

Art. 5. Once the resignation is presented, the office relative to articles 1-3 will be extended until acceptance of the resignation is communicated to the interested party, for a fixed or unspecified time, contrary to the general terms established by canons 189 §3 cic and 970 §1 cceo.

Canon 402. §1 A Bishop whose resignation from office has been accepted, acquires the unless, because of special circumstances in certain cases, the Apostolic See provides otherwise.
[NB see Authentic Interpretation of canon 402 §1, 10.X.1991]

§2 The Episcopal Conference must ensure that suitable and worthy provision is made for the upkeep of a Bishop who has resigned, bearing in mind the primary obligation which falls on the diocese which he served.