I am a widow and not to good at fixing things. I got a fIlter for my furnace 16x20.But I am not sure how to put it in. The slot is on the upper right side of the horizontal unit. They said they changed it but after pulling it out I know they didn't. The filter looks the same on both sides. Can someone help me? Thanks

Locate the arrow on the filter and point it toward the unit. If no arrow is present then airflow direction doesn't matter.