Disable dimmer switch
A key question, as noted in the comments, is whether this is a real 3-way setup, where either switch can be used at any time to turn the light on or off, or some sort of &quot;crazy thing&quot; (like in the dining room I grew up with - timer replaced one 3-way switch and we had to always remember to leave the other switch &quot;on&quot;).
Assuming this is a proper 3-way switch, with one of the switches being the dimmer:

Turn off power at the breaker
Open up the dimmer switch. You should find 3 wires (plus possibly a green or bare ground wire).
Determine which wire is common (which may be hot or switched hot, but doesn't actually matter much) and which two wires are travellers. How you do that will vary depending on the setup. If the wires attach to screws on the dimmer then usually two screws will be one color and the 3rd screw will be a different color. The screws with the same color should be the travellers. Or the wires may actually be labelled on the dimmer. But color (except green for ground) is no guarantee of anything (unless wires are part of the dimmer and labelled on the dimmer). If it isn't 100% clear, upload pictures before disconnecting any wires.
Remove the dimmer and install a simple 3-way switch (you can get regular or Decora, plus get a face plate to match). The new switch will come with instructions that should make it very clear which wires or screws are for the travellers.
Put everything back together and turn on the power.