How To Smooth

The textures are 4096x4096 and highly detailed however it might be that you do not want it to be super detailed. To easily smooth them you can open the texture and increase the LOD Bias. This will decrease the displayed resolution.

LOD Bias 1 will divide by 2

LOD Bias 2 will divide by 4

LOD Bias 3 will divide by 8

LOD Bias 4 will divide by 16

LOD Bias 5 will divide by 32

LOD Bias 6 will divide by 64

Personally I wouldn't recommend going past LOD Bias 2 or 3

Alternatively you could export the stamp layer to your landscape layer so you can use a smoothing brush instead. More about this in the link below.

Exporting Stamp Layer - Saving Memory