2, 3, 12, 14, 15, 16, 19, 24, 27 and NM1: diversions due to the Piazza Cordusio redevelopment 

From June 30 to September 14, several lines will change their service. Track renewal works will be carried in via Mazzini as part of piazza Cordusio regeneration project.
If useful for your trip, consider the metro as an alternative.

 MAP OF THE NEW ROUTES

Tram 2. No service between Colombo/Coni Zugna and Lanza

Normal service between piazzale Negrelli and via Colombo/viale Coni Zugna (Porta Genova) stops. Trams are diverted between Lanza and Bausan via piazzale Baracca, largo V Alpini, Cadorna and Cairoli.
Between Lanza and Porta Genova, use M2. Between Conciliazione and Duomo, use M1. Between Coni Zugna and De Amicis, use M4.
Attention. From July 19 to September 7 M2 is closed between Cadorna and Garibaldi, we’ll be upgrading the rail infrastructure and replacing the tracks (info here). 

Tram 3. No service between Duomo and San Gottardo/piazza 24 Maggio stop

Normal service between Gratosoglio and Corso San Gottardo/piazza 24 Maggio stop. Then they are diverted and follow the usual path of tram line 14: via Gorizia, via Vigevano, Porta Genova, via Colombo, viale Coni Zugna, via Solari and Lorenteggio. To go to the centre, use M2 from Abbiategrasso or Porta Genova, or M4 from Coni Zugna.

Tram 12. No service between Missori, Lanza and Certosa

Normal service between viale Molise and piazza Missori stop. Then trams follow part of the usual path of the tram line 16 until Monte Velino. Between Lanza and Certosa, use the diverted tram 16. Between Missori and Cadorna use M3+M1 (changing at Duomo). Between Cadorna and Lanza, use tram 2. Between Certosa and Roserio, tram 12 is replaced by bus B12.

Tram 14. No service between Lorenteggio, Cantore and Cairoli (via Cusani)

It runs as usual between Cimitero Maggiore and Cairoli (via Cusani). Between Lorenteggio and Cantore it is replaced by tram 3. To go to the centre use M4 from San Cristoforo or Coni Zugna, or the M2 from Porta Genova.

Tram 15.

Trams terminate at piazza Fontana, not at via Dogana.

Tram 16. No service between largo D'Ancona, Missori and Monte Velino.

Normal service between San Siro and Largo D'Ancona. Then trams are diverted via Cadorna, Cairoli, and Lanza. After Lanza, they follow the usual path of tram line 12 until Certosa. Between Missori and Monte Velino, use the diverted tram line 12. Between Cadorna and Cordusio, use M1 or tram 1.

Tram 19. No service between Bixio and piazza Virgilio

Trams are diverted in both directions and skip the stops between viale Piave/via Bixio and piazza Virgilio. When diverted, trams stop at viale Piave, piazza Oberdan (Porta Venezia M1), viale Vittorio Veneto, piazza Repubblica, via Turati, via Manzoni, via Santa Margherita, via Tommaso Grossi, via Broletto, via Cusani, Cairoli and Cadorna.
Between Porta Venezia and Cordusio, use M1. Between Repubblica and Duomo, use M3.
Between via Bixio and piazza 5 Giornate, use tram line 9, and then tram line 12 to Missori.

Tram 24 and 27. They merge into a single line: 24

Tram 27 is replaced by tram 24 (read below).

Tram 24. It runs between Vigentino, via Larga and viale Ungheria

Trams run as usual between the Vigentino terminus and via Larga stop. From via Larga, they follow the usual path of tram line 27 until viale Ungheria.

NM1 Night Bus

Buses from Molino Dorino to Sesto FS don’t stop at via Carducci (Cadorna), via Meravigli, via Dante (Cordusio), via Orefici (Duomo), piazza Diaz (Duomo), via Larga. Towards Molino Dorino, buses skip the stops of via Larga, via Gonzaga, via Mazzini, via Orefici, via Dante, via Meravigli, Cairoli and via Carducci. As an alternative, use bus 85 stops at Cairoli and largo Augusto (San Babila).