Capacitor Ratings and Good Sustitutes

Generally in electronics circuits repair you can increase voltage rating but not the capacity.and you can not decrease any of these.

Also polarity marking means its a DC capacitor not AC.

I concur. With caps you can generally increase voltage rating but I wouldn't go beyond the next step up simply because of circuit protection. In addition, the farad rating must stay the same or you change the circuit, causing additional failure, possibly catastrophic. Another addition, it is generally important to change like for like, i. e. electrolytic for electrolytic. Wafer caps or otherwise will work but will not take the same stresses as OEM. BTW, yes these are DC caps.

Electros failure can be hi esr, s/c, leaky,o/c,reduced cap- etc. have you tested your electros for any of this, or are just blanket r/p?. If latter- go for it- use quality brand electros- if possible use 105'C + electros- to meet makers specs should r/p with exactly same volt/cap- but I have found no harm in using say in your case 25-50v, cap about 25% higher- never lower,&observe correct polarity.

You will almost certainly find that the capacitors will have a dc voltage across them when the circuit is working properly. The minimum voltage rating required is this dc voltage plus the peak of any ac voltage. Note the capacitors need to be in the right way round. You should be able to identify polarity markings on the capacitors you are taking out and the ones you are putting in as replacements

You've asked a very naive question that makes me wonder two things. First, why you wish to replace these components? Second, will you be capable of a successfully replacing these components without damage to the circuitry?

But to answer your question, the voltage rating on a capacitor is the DC blocking or hold-off voltage that the capacitor is designed to withstand for its lifetime. All capacitors have this parameter. Most electrolytic capacitors will survive brief surges of 150% of the continuous hold-off value. Most electrolytic capacitors also have a DC polarity bias that must be adhered to for anticipated longevity. The original designer choose a capacitor with a larger hold-off voltage than the DC voltage present on the capacitor. This is why you've heard the comment that when replacing a capacitor you can increase the voltage rating but not the value. The original circuit designer should have chosen the hold-off value of the capacitor to maximize the lifetime of the circuitry without sacrificing space or cost. (A 50V electrolytic capacitor should hold off 5V longer than a 6V rated capacitor. But if the circuit is not expected to last longer than a few years this will just cost more for no value.) There are also a myriad of other factors in capacitor selection beyond the scope of your question.