There are two ways to "weld" vertices: Merge or Remove Doubles.

Merge will take 2 verts and basically make one out of them at a half-way point between the two.
Slow.

Remove Doubles will take selected adjacent verts and make them into one. How adjacent they are will decide how accurate this will work. Under Mesh Tools (after you press F9 in the button bar region) you will see a Limits toggle thingie: that decides how close verts have to be before they get made into one vert.

Try the second one (Remove Doubles) first... remember, you can always hit Ctrl Z and try Merge. Only, with Merge, it's vert pair by vert pair. As I said: Slow.

ETA: it would be either W -> 5 or W -> 6. That's in Blender 2.49b.

Monterey/Mint21.x/Win10 - Blender3.x - PP11.3(cm) - Musescore3.6.2

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