There are a couple of spells and a potion that may help in this.
The useful spells are:
You touch a nonmagical weapon. Until the spell ends, that weapon becomes a magic weapon with a +1 bonus to attack rolls and damage rolls.
A nonmagical weapon you touch becomes a magic weapon. Choose one of the following damage types: acid, cold, fire, lightning, or thunder. For the duration, the weapon has a +1 bonus to attack rolls and deals an extra 1d4 damage of the chosen type when it hits.
Beside the +1 bonus to attack rolls and/or damage (and the further elemental damage given by the latter), the weapon becomes magical for the purposes of hitting creatures that have resistance to non magical damage.
If no one in the party has one of these spells, another option is the Oil of Sharpness (emphasis mine):
This clear, gelatinous oil sparkles with tiny, ultrathin silver shards. The oil can coat one slashing or piercing weapon or up to 5 pieces of slashing or piercing ammunition. Applying the oil takes 1 minute. For 1 hour, the coated item is magical and has a +3 bonus to attack and damage rolls.
There is though a limitation on the type of weapon: it works only on slashing or piercing ones. If you summon a camel, for example, this won't work.
The above options can be used on natural weapons: indeed the MM (page 9-10) says (emphasis mine)
The most common actions that a monster will take in combat are melee and ranged attacks. These can be spell attacks or weapon attacks, where the "weapon" might be a manufactured item or a natural weapon, such as a claw or tail spike.
hence there are no problem casting the meniotned spells or using the Oil of Sharpness on the steed's weapons. This is confirmed also by SAC:
Are natural weapons considered weapons? Things designated as weapons by the rules, including natural weapons, are indeed weapons. In contrast, unarmed strikes are not weapons. They are something you do with an unarmed part of your body.
Check also this question and relative answer here (and other useful Q&A linked there in the comments).