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I'm not sure you should do that. From the photo it appears to be a hybrid like 'Alta' that was propagated to achieve that columnar form. If you head it back, even a few inches, you will distort the natural form, and may not achieve what you intend to. If it is what I think it is, it won't take readily to spreading out too much. If you are not sure about the variety, maybe wait and see how it wants to grow would be a good approach.

This looks like sun damage or sun scald to me. When a plant moves from indoors, or even a greenhouse; it's best to harden off the plant. This means moving the plant to the outdoors and letting them adjust to the outdoor sunlight conditions slowly, over a period of a few days to a week.
Shade the shrub from the hottest part of the day until it is established.
This shrub does best in full sun to partial shade. It requires an evenly moist well-drained soil for optimal growth, but will die in standing water. It is not particular as to soil type, but has a definite preference for acidic soils. Consider applying a thick mulch around the root zone in winter to protect it in exposed locations or colder microclimates.

This is outside of my expertise.

It would appear that you have a deciduous magnolia. Do you know the specific cultivar name? I see new leaf growth starting, which will happen after flowers have dropped on deciduous varieties.

This article will help: https://www.gardeningknowhow.com/ornamental/trees/magnolia/which-magnolias-are-deciduous.htm

There is only one way to tell how old a magnolia tree is, and that is by counting the tree's rings. Each year the tree puts on a new layer of growth, creating a thin dark ring between the new growth and last year's growth. By counting these rings you can determine how many growing seasons the tree has seen--in other words, how old the tree is. The trick is to find a way to count the rings without cutting the tree down to reveal them. Fortunately there is a way to accomplish this. It is a device called an increment borer.

Check with a certified Arborist for this device.

A number of problems are potential culprits when magnolia buds (either for flowers or for leaves) are not opening. One possibility is that thrips are attacking your magnolia buds. But weather comes to mind, first and foremost, as a possible reason for the failure. What kind of spring weather did you this year? Magnolia buds can rot during extended periods of such weather, and once rot sets in, the buds will not open.
This article will help you.

https://www.gardeningknowhow.com/ornamental/trees/magnolia/magnolia-blooms-not-opening.htm