When describing an upward slope or angle, there are several synonyms you can use for the phrase "slanting up." Some options include angling upwards, ascending, climbing, inclining, rising, sloping upwards, and tilting upwards. Each of these phrases gives a slightly different nuance to the description, so consider the context in which you're using the term. For example, if you're discussing the ascent of a mountain or hill, "climbing" or "ascending" might be the most appropriate synonym. However, if you're describing a gradual incline, "sloping upwards" or "tilting upwards" may be more fitting.