My Apple Watch series 1 won’t charge

I can’t find a answer anywhere but I have a Apple Watch series 1 that has not been updated in a while or used so I bought a Apple Watch charger for it a few days ago (I lost my other one) and I connect it to my watch that has been Reset cause I bought a new phone and when I connect it to the charger it will power on to the language screen then when I tap english it says power low so I will take the Watch off and put it back on charger and it works for about 1.5 seconds and this message comes up and says to use this accessory (thecharger ) upgrade watchos but how am I suppose to update it if it won’t charge please actually helpful answers only