How to change stroke thickness

i created an outline shape using the offset curve and the stroke thickness is very thin, how can I adjust it without drawing a new shape again?

Are you using Glyphs 3? I would advise you to use the Stroke function (select a path and you will find a stroke option in the bottom-right corner of your screen), as it is not destructuve to your path and you can easily adjust both the stroke as well as the base path.

If you have converted a path into a stroke with the Offset Curve filter, you can only undo the action, there is no way of 100% re-attaining the original path (other than some rather hacky ways). Always keep a backup of the original path in the background for exactly this reason.