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You have shaded smooth your object but the topology is bad (ngons and stretched triangles), therefore these artefacts. But it depends on what you want to do at the end, if you don't care about having a perfect geometry and you want a quick fix in order to have a correct appearance, use the Auto Smooth option in the Properties panel > Object Data > Normals. It will smooth all the angles below the angle you'll set and keep the other ones sharp, therefore it will remove all these artefacts:

You have shaded smooth your object but the topology is bad (ngons and stretched triangles), therefore these artefacts. But it depends on what you want to do at the end, if you don't care about having a perfect topology and you want a quick fix in order to have a correct appearance, use the Auto Smooth option in the Properties panel > Object Data > Normals. It will smooth all the angles below the angle you'll set and keep the other ones sharp, therefore it will remove all these artefacts:

You have shaded smooth your object but the topology is bad (stretched triangles), therefore these artefacts. But it depends on what you want to do at the end, if you don't care about having a perfect geometry and you want a quick fix in order to have a correct appearance, use the Auto Smooth option in the Properties panel > Object Data > Normals. It will smooth all the angles below the angle you'll set and keep the other ones sharp, therefore it will remove all these artefacts:

You have shaded smooth your object but the topology is bad (ngons and stretched triangles), therefore these artefacts. But it depends on what you want to do at the end, if you don't care about having a perfect geometry and you want a quick fix in order to have a correct appearance, use the Auto Smooth option in the Properties panel > Object Data > Normals. It will smooth all the angles below the angle you'll set and keep the other ones sharp, therefore it will remove all these artefacts:

It depends on what you want to do at the end, if you don't care about having a perfect geometry and you want a quick fix in order to have a correct appearance, use the Auto Smooth option in the Properties panel > Object Data > Normals. It will smooth all the angles below the angle you'll set and keep the other ones sharp, therefore it will remove all these artefacts:

You have shaded smooth your object but the topology is bad (stretched triangles), therefore these artefacts. But it depends on what you want to do at the end, if you don't care about having a perfect geometry and you want a quick fix in order to have a correct appearance, use the Auto Smooth option in the Properties panel > Object Data > Normals. It will smooth all the angles below the angle you'll set and keep the other ones sharp, therefore it will remove all these artefacts: