Installing Currency Assistant is very easy.
Double-click the Currency Assistant 3.6.3.dmg file to mount the Currency Assistant 3.6.3 disk image.
Open the mounted volume and drag the Currency Assistant icon to the Applications folder.
If you are replacing an old version and macOS tells you that the operation can’t be completed because you don’t have permission to access some of the items, make sure the old version is not running and move it to the Trash. Then drag again version 3.6.3 to the Applications folder.