Electrolytic caps can dry out over time, or be damaged by over voltage excessive temperatures or excessive current draw in that circuit, the tops of the caps should be nice and flat, one that is bulged up is or will be going bad.
I tend to replace the cap with one that has a slightly higher voltage rating, this will keep it from being damaged again.
If you have an oscilloscope or a good DVM you can check for excessive AC ripple in that circuit, (it should be less then 200 mV AC) and also what the DC voltage is on that circuit, if within 10% of the caps rated voltage you may have a DC voltage regulator problem in that circuit.
In most power supply circuits the caps rated DC voltage should be one and a half to two times the DC voltage needed for that design.
If that circuit is running at 3 VDC and the cap is rated for 6.3 it's a safe design, if the circuit is rated at 6 volts DC and the caps only rated to 6.3 it's not a good idea.