Replacement of component from which you will be lowered down Fig. 6
If you are dealing with a rusty, not suitable or worn out carabiner (or "maillon rapide"), you can replace it with a certified "maillon rapide " (this is the cheapest solution), or with a carabiner, or with a quickdraw (in this last case it's better to set the carabiner of the quickdraw in the opposite direction of the element that has to be reinforced, provided that it does not give rise to a leverage effect on the rock).
Making a belay "to leave there" Fig. 5
It's a good choice if the anchors (that is bolts and hangers) are still in good condition and the link between them, instead, is compromised and not suitable (rusty chain or rotted rope). This option requires that you are equipped with a rope or a sling and a "maillon rapide".
If the belay is so compromised or dangerous that an eventual reinforcement is not enough to ensure an adequate level of safety (for example a belay that has undergone a rockfall or with the anchors completely rusted) you can anyway adopt some strategies: after an eventual reinforcement, arrange your lowering down doing a "Self-locking Machard knot", Fig. 7, with a lanyard (possibly made of kevlar) on the branch of rope that goes from the belay to the belayer, and tighten the knot with your hand during the lowering down to allow for its sliding. In doing so, if, unfortunately, the belay would give way, the Machard knot will shorten the length of the fall as the one of the fall from the closest anchors, provided that the anchors of the route are suitable, and except for the first anchors of the route (because if the belay cracks when you have already passed the first anchors, there is no anchor from which you can hang...).