To get into E+ you either need to buy the access or be any UA elite level.

Buying can happen with the annual E+ access from the UA website or you can buy up segment-wise upon checking in.

You can also get the lowest UA frequent flyer level Premier Associate from a UA elite.

If the flight is not full you can also poach an E+ seat. Chances are <50% that an FA will send you back.

If the flight is full, you can check in late and hope for all E- gone by that time in which case they would move you to E+ for free.