This can be done quickly with the help of a boost:
https://news.blizzard.com/en-gb/diablo4/24158873/return-to-s...Go to the city of Kyovashad.
From the waypoint, go North and enter the inn. On the left is Fated Reliquary, interact with it to boost your character to level 50.
Here it is on a map:
https://mapgenie.io/diablo-4/maps/sanctuary?locationIds=2709...I had to create a new character without the "seasonal" option:
https://us.forums.blizzard.com/en/d4/t/wheres-the-fated-reli...And I chose to
skip the campaign when creating the character.
Please note that it can be used only once, I'm new to this game so I don't know if it should be used with care. Here is one article that explain seasonal and eternal characters:
https://www.polygon.com/diablo-4-guides/23800830/seasonal-et...