Whether you're inheriting an engagement ring from his family or yours there are number of things to consider and do:
1. Decide if you like the ring as is. Maybe it's not really your style. Maybe it's round and you always wanted a pear-cut diamond. If you love the ring as is great! If not, you can always take the stones and have it reset or you can always decline the ring and pass it to a sibling or cousin.
2. Have a jeweler determine if it has to be reset. Depending on how old the ring is the prongs holding the stone may need to be tightened or the setting may be too loose to salvage, in which case you can reset it the stone(s) in the same design or create a new setting.
3. Make sure it's always insured. Before you or your fiance take possession of the ring make sure that you have it added to your home owner's insurance policy.
4. Make it yours. Even though it's touching and important that you are receiving a family heirloom it's important that you love the ring and want to wear it everyday. Don't be afraid to change the setting to something that fits your style. You could add side stones or choose a band with diamonds.