Add some compression to your recordings. You can use a hardware compressor unit or a software plugin inside your recording software. Experiment with different presets and settings until you achieve a warmer sound.
Move your mouth closer to the mic if you are singing, or move your instrument closer and see if you get a warmer sound while recording. Some microphones have a "proximity effect" that makes the sound louder and warmer as you get closer to it.
Apply some EQ (equalization) effects with a hardware EQ or a software plugin inside your recording software. Many software EQ plugins have presets such as "warmer" and "rounder" that will boost the mid-range of the sound and create a warmer tone. Experiment with different settings until you achieve the sound you would like.
Record with multiple microphones to find one with a warmer sound. Different microphones are designed for different purposes, and some may sound warmer than others.
Set up sound absorption panels in your recording space to absorb sound. These panels are made of a sound-absorbing material to prevent sound reflections. This allows you to remove the sound of the room altogether.