Here's what you should do to ensure using it won't damage anything. First make sure the output voltage is the same - if it is not, do not use it. If the output voltage is the same, use it only to charge the laptop battery but do not attempt to run the laptop on the power adapter power, unplug it first. Note that it will take quite a bit longer to charge than before.
Hope it helps
If its the same voltage, then you can use it. But if you are using more power then it can provide, your laptop will start drawing from the battery as well as from the power brick. So try not to do anything that use more than 65w (like gaming, which will definitely max out the power draw and go over 65w) if you want continuous usage. Battery will also take longer to charge, which is not necessarily a bad thing (using less amp and longer time to charge a battery can prevent overheating). Keep your laptop on the low power profile for the next two week until the 90w power brick arrive.
never do this please, the 90W adapter has a dfferent plug connector with the 65W adapter, if the original power adapter is 90W, never use a 65W one, but if your laptop uses a 65W power adapter, you can use a 90W power adapter to replace it...