For a photograph this old, the safest advice is to consult a professional photo or paper conservator before trying to remove or repair anything yourself. An old cardboard mount may be non-archival and can actively damage the print over time.

General preservation steps:

• Keep it away from direct sunlight.
• Avoid high humidity, which can encourage mold.
• Keep it away from wood, rubber, degrading adhesives, insects, and rodents.
• Do not simply seal the photo together with the old cardboard in a bag; that can trap it with damaging materials.

If the print is ever separated from the old mount, old photographic prints can curl badly. One community-suggested storage method is to place the print in a clear polyester archival sleeve/envelope, then support it with archival mat board if framing or storing.

A good first step is to ask a reputable art framing shop for a referral to a conservator. With fragile historical photos, professional handling is usually the best way to prevent further loss.