In the console you type "map <mapname>"
Here are some map names:
d1_canals_01, d1_canals_02, d1_canals_end, d1_tempanals_02, d1_town_01,
d1_town_02, d1_town_03, d1_town_04, d1_town_05, d1_trainstation_01,
d1_trainstation_02, d1_trainstation_03, d1_trainstation_05, d1_under_01,
d1_under_02, d1_under_03, d1_under_04, d2_coast_01, d2_coast_02,
d2_coast_03, d2_coast_04, d2_coast_04_dx60, d2_coast_05, d2_coast_06,
d2_coast_07, d2_coast_08, d2_prison_01, d2_prison_02, d2_prison_03,
d2_prison_04, d2_prison_05, d3_c17_03, d3_c17_04, d3_c17_05, d3_c17_06a,
d3_c17_06b, d3_c17_07

You can also type "maps" and it lists the maps.

DaveL

Open the Play Games menu, right-click on Half-Life 2, select Properties,
click on Launch Options and paste this into the box:

+sv_unlockedchapters "15"

That will unlock all Half-Life 2 chapters. You can adjust the number to
whatever you want (obviously not above 15).