A bond is a debt with a specified interest rate, and a specified term.
It must be repaid at the end of the term.
Typically, a new bond is issued at your present interest rate to repay the old bond [refinance]. That is - a new bond replaces the old bond, and the interest rate reflects your current creditworthiness.
This is done 'automatically,' when a bond expires, regardless of any lock setting you have on the bond panels.
You may alternately, repay a bond outright - that is, take your surplus cash and pay the bond off, without replacing it.
Tip - if you are planning to repay one of many bonds, repay the one with the longest expiration and/or the highest interest rate. Pay attention to your credit rating. If you want to know what the present rate really is, go through the 'issue bond' sequence to see a proposed bond, and then cancel it [don't issue the bond]. For instance, you have an 8% bond that expires in three months, and a 7.5% bond that expires in a year, and your present credit rating will garner you 6.5% one-year bonds. Pay off the 7.5%-OneYear bond and let the 8%-ThreeMonthsLeft bond be refinanced at your better rate in three months time. There may be a lot of complex accounting to do to assess a complex bond portfolio for the best candidate. Good thing it's a game: Don't worry - be happy.
Other tips - if your credit rating has significantly improved, AND you have credit available to issue bonds at will at a size large enough to replace an existing bond, you might start your own renewal program. That is, you issue a new bond [at a better interest rate] and immediately pay off an old bond [one with the highest interest cost]. You can typically do about one of these a day. Be careful to space these out, as you do not want to have a portfolio of bonds that all come due in a series of days a year out.
Colorless green ideas sleep furiously [but otherwise, they do not worry and are happy].