In this video, I describe and show how to change strings on an upright double bass instrument. Several important thing that should be considered: First of all, don't change all strings at once, as this could damage your instrument if the sound post falls or changes position. So please, only change the strings one after another. Secondly, mark the position of the bridge; it may change position during string change, but it shouldn't. Thirdly, apply some graphite to the slots of the bridge and saddle to guarantee a smooth sliding of the strings while tuning. You might use a chromatic tuner for tuning the strings, but you should not overtune them.
When done, be happy with your new stringed double bass!