There are two methods of changing the the apperence of a actor/ the model itself. It greatly depends where you want to change it.

1. If you just want to change it just once it works just fine in the Level it self here "On Static Mesh itself"
2. If you want to change it on every Actor that is connected with a particular Blueprint then use "In a Actor Blueprint"

On Static Mesh itself

Models

1. Click on the Mesh in the Viewport (big Window with a preview of the level in the middle)

2. Find the Static Mesh Headline in the Details Tab

3. Drag a Static Mesh/ Model from the Content Browser to the Picture of the current static mesh

   or click on the Name of the current static mesh and browse through the names until you find the right one

Materials

1. Click on the Mesh in the Viewport (big Window with a preview of the level in the middle)

2. Find the Static Mesh Headline in the Details Tab

3. Drag a Material from the Content Browser to the Picture of the current Material

   or click on the Name of the current Material and browse through the names until you find the right one

Problems that could occur/ stuff that you don't find by "On Static Mesh itself"