升天 literally means 'to ascend to heaven'. It is a euphemism for dying, commonly used in both general and religious contexts (especially Christianity for the Ascension). As a separable verb (离合词), you can insert aspect markers or complements between the syllables: 升了天 ('passed away'), 升过天 ('has ascended'). Avoid using it for everyday death—use 去世 or 死了 instead unless you want a poetic or religious tone.