To find an upper bound estimate of a certain quantity means to find a number that is surely greater than the quantity we're estimating. So, you're saying something like "I don't know the exact value of this expression, but it's surely no more than..."
The lower bound estimate is exactly the opposite: you're giving a number that is surely less than the quantity you're estimating, so it is described by a sentence like "I don't know the exact value of this expression, but it's surely no less than..."
So, if you want an upper bound estimate for 354 + 267, we could work like this: 354 is less than 360, while 267 is less than 270. This means that their sum can't be larger than 360+270 = 630. So, 630 is an upper bound for 354 + 267
Similarly, 63.75 is more than 60, and 3.63 is more than 3. So, 63.75 + 3.63 can't be smaller than 60 + 3 = 63. So, 63 is a lower bound for 63.75 + 3.63
