Record a spectrum of an Al-mirror and then from your sample. Then divide the sample spectrum by the spectrum of the Al-mirror (keep in mind that the reflectance of the Al-mirror is somewhat smaller than one; if you want precise absolute reflectance values, you need to correct for that).

Dear Hannah, it is important also, how was your data sat measured. For example, if was used accessory with variable angles, then you can follow Thomas recommendation. If was used absolute reflectance accessory (VW etc.), then the correction is included in your raw data. If was used 45/0 or inverse geometry or integrating sphere it is necessary to measure raw data of standard, which is near to PRD (perfect reflecting defuser) such as Labsphere Spectralon or pressed Barium Sulphate and these raw data relate to known absolute reflectance data of these standards.